Day of the Week Calculator
Enter any date to find its weekday. The result also shows where the date falls in the year, its ISO week, and the calendar convention used — plus the month around it, the same date in nearby years, and, for dates before the Gregorian reform, what the calendar actually said in a particular country.
Find the day of the week for any date
Thursday
September 3, 2026
Calculated on the proleptic Gregorian calendar.
- Day of week
- Thursday
- ISO weekday number
- 4 Monday is 1, Sunday is 7
- Day of year
- 246 of 365 January 1 is day 1, so this date is counted
- Days remaining in year
- 119 after this date, not counting it
- Days elapsed before it
- 245 before this date, not counting it
- ISO week
- W36 of 53 ISO week date 2026-W36-4
- ISO week-year
- 2026 same as the calendar year
- Weekday or weekend
- Weekday Saturday and Sunday counted as the weekend
- Leap year
- No 2026 has 365 days on this calendar
- Quarter
- Q3
- Day in month
- 3
- Days in month
- 30 September 2026
- Ordinal weekday in month
- the 1st Thursday of 4 in September
- Ordinal weekday in year
- the 36th Thursday of 2026 of 53 in the year
- Same weekdays remaining
- 17 more Thursdays left in 2026
- Calendar convention
- proleptic Gregorian
How this result was calculated
| Input date | 2026-09-03 |
|---|---|
| Calendar | proleptic Gregorian |
| Leap year | No |
| Day of year | 246 of 365 |
| Day index | 739862 (Rata Die: day 1 = 0001-01-01) |
| Weekday index | 4 ((739862 − 1) mod 7) + 1 |
| Weekday | Thursday |
| ISO week | 2026-W36-4 |
| Engine | CalcDomain calendar engine v1.0.0 |
Verified by CalcDomain calendar engine v1.0.0. Every row above is read from the same engine state that produced the answer, not recomputed for display.
Month calendar
| Mon | Tue | Wed | Thu | Fri | Sat | Sun |
|---|---|---|---|---|---|---|
Around this date
- Previous day
- Wednesday, September 2, 2026
- Next day
- Friday, September 4, 2026
- Previous Thursday
- August 27, 2026
- Next Thursday
- September 10, 2026
- Same date last year
- Wednesday, September 3, 2025
- Same date next year
- Friday, September 3, 2027
Today, and dates relative to it
Today's date is read from your own device's calendar when this page loads, so it is never baked into the page at build time. Enable JavaScript to see it.
Relative date
Next or previous weekday
The same date across nearby years
| Year | Date | Weekday | Leap year |
|---|---|---|---|
| 2021 | September 3, 2021 | Friday | No |
| 2022 | September 3, 2022 | Saturday weekend | No |
| 2023 | September 3, 2023 | Sunday weekend | No |
| 2024 | September 3, 2024 | Tuesday | Yes |
| 2025 | September 3, 2025 | Wednesday | No |
| 2026 centre | September 3, 2026 | Thursday | No |
| 2027 | September 3, 2027 | Friday | No |
| 2028 | September 3, 2028 | Sunday weekend | Yes |
| 2029 | September 3, 2029 | Monday | No |
| 2030 | September 3, 2030 | Tuesday | No |
| 2031 | September 3, 2031 | Wednesday | No |
The calendar actually in force
Which calendar was running on a given date depends on where you mean, so this mode needs a jurisdiction. Only jurisdictions with a cited transition record are offered — this page does not hold an all-country historical calendar and does not pretend to.
This civil date did not exist here
September 5, 1752 never existed in Great Britain and the British colonies. The calendar jumped from September 2, 1752 (Old Style, Julian) straight to September 14, 1752 (New Style, Gregorian), omitting 11 nominal days. No day of the week can be given for a date that was never used.
- Last Old Style date
- Wednesday, September 2, 1752 Julian
- First New Style date
- Thursday, September 14, 1752 Gregorian
- Nominal days omitted
- 11 September 3, 1752 to September 13, 1752
Those two dates are consecutive physical days: the civil labels jumped, but no time was lost and no weekday was skipped. No day of the week is given above, because none was ever used.
The same civil label under each convention
| Convention | Result | What it means |
|---|---|---|
| Historically observed — Great Britain and the British colonies | did not exist | The calendar actually in force there on that date, from a cited transition record. |
| Proleptic Gregorian | Tuesday | Modern Gregorian rules projected backward. A convention, not a historical record. |
| Julian | Saturday | Julian leap-year rules, as most of Europe used before the reform. |
These are three different questions, not three attempts at one. A calculator that shows only one of them, without saying which, is the reason two tools can disagree about an old date and both be right.
What day of the week is it today?
Today's weekday is not written into this page. It is worked out in your browser from your device's own calendar the moment the page loads, then handed to the same engine every other answer here comes from. That is deliberate: a "today" baked in when the page was built would be wrong within a day, and a "today" resolved on a server would be the server's date, not yours.
Because it is your device's local civil date, it is the one figure on this page that can legitimately differ between two readers. Someone in Auckland and someone in Los Angeles genuinely are on different calendar dates for part of every day. The Today tab names the time zone it used, so you can see which local calendar produced the answer.
How the calculator works
A date you type is a civil date: a plain year, month and day. It carries no time of day, no timestamp and no time zone, which is exactly why an entered date resolves to the same weekday wherever you open the page — there is no instant for a time zone to shift. Before anything is calculated, the date is validated against the real length of its month on the calendar you chose. February 30, April 31, month 13 and February 29 in a common year are all rejected outright. None of them is quietly moved to a neighbouring real date, which is what JavaScript's own Date object would do: new Date(2026, 1, 30) silently becomes March 2.
How is the day of the week calculated?
Weekdays can be calculated by converting a civil date to a continuous day count and reducing that count modulo seven. Named methods such as Zeller's congruence and Conway's Doomsday rule are alternative ways of obtaining the same weekday — different routes, one destination. None of them is mandated by any standard; ISO 8601 specifies how dates and week dates are represented, not which arithmetic you must use to derive them.
This page uses a documented integer day index: a Rata Die day number in which 0001-01-01 on the proleptic Gregorian calendar is day 1, and a Monday.
dayIndex = daysBeforeYear(y) + daysBeforeMonth(y, m) + d
isoWeekday = ((dayIndex − 1) mod 7) + 1
Every other figure — the ISO week date, the day of the year, the ordinal occurrence of a weekday in its month and year, the relative dates, the neighbouring days — is integer arithmetic on that one number, so no two values on the page can come from parallel formulas and disagree. The test suite cross-checks Gregorian results against a separately implemented algorithm (Sakamoto's) and Julian results against Zeller's congruence, over ten thousand deterministic pseudo-random dates each, and requires zero mismatches. The check is meaningful precisely because those algorithms share no code with the production engine.
Worked example
Take January 1, 2000, on the proleptic Gregorian calendar. Its day index is 730120. Subtract one and divide by seven: 730119 mod 7 = 5. Counting Monday as 1, remainder 5 is weekday 6 — Saturday. It is day 1 of a 366-day year, so 365 days remain after it, and 2000 is a leap year because it is divisible by 400. Its ISO week date is 1999-W52-6: the ISO week-year is 1999, not 2000, because the Thursday of that week fell in December 1999. It is the first Saturday of January and the first of the year, with 52 more to come. Every figure in this paragraph was produced by the shipped engine when this page was built, and is frozen as a golden vector in the test suite.
Worked example: a date that never existed
Now ask for September 10, 1752 with the jurisdiction set to Great Britain. The calculator does not return a weekday. It returns a dedicated state: this civil date did not exist here. The Calendar (New Style) Act 1750 enacted that the calendar would run as before until 2 September 1752 inclusive, and that "the natural day next immediately following the said second day of September shall be called, reckoned, and accounted to be the fourteenth day of September, omitting for that time only the eleven intermediate nominal days of the common calendar." So Wednesday, September 2, 1752 was followed directly by Thursday, September 14, 1752. September 3 to 13 are not dates with a forgotten weekday; they are labels nobody in Britain ever used, and inventing an answer for them would be fiction. Note also that no time was lost in the reform — the two dates either side have consecutive day indices (639796 and 639797), and the seven-day week ran through the change unbroken.
Gregorian and Julian: why the same date gets two weekdays
The two calendars differ in exactly one rule. The Julian calendar makes every fourth year a leap year, without exception. The Gregorian calendar removes three of those leap days every four centuries by making century years common unless they are divisible by 400 — so 1900 and 2100 are common years while 2000 and 2400 are leap years. The Julian rule overshoots the solar year by about 11 minutes, which compounds to roughly three days every four centuries. That accumulated drift is the whole reason the reform happened, and it is why a Julian and a proleptic Gregorian reading of the same old date land on different weekdays.
Neither reading is "the true date" on its own. A pre-reform date written in a contemporary English document is almost certainly Julian; the same date processed by modern software is almost certainly proleptic Gregorian. Both are correct answers to different questions, and the only real error is failing to say which one you used. This page always says.
ISO weeks: Monday is weekday 1, and the week-year is not the calendar year
ISO 8601 defines a week date. Under it Monday is weekday 1 and Sunday is weekday 7, and week 1 is the week containing 4 January — equivalently, the week that holds the year's first Thursday. Because a week belongs entirely to whichever year contains its Thursday, the ISO week-year can differ from the calendar year at both ends of the turn: 1 January 2016 sits in week 53 of week-year 2015, and 31 December 2019 sits in week 1 of week-year 2020. A year has 52 or 53 ISO weeks, and the result above tells you which.
Two things are worth separating here. The ISO weekday numbering used in every calculation on this page always starts at Monday. Which day a calendar grid starts on is a different question entirely — a locale convention that varies by country, recorded in reference data such as Unicode CLDR. You can switch the month calendar above between Monday and Sunday, and it will change which column a day sits in and nothing else. ISO 8601 does not claim Monday is universally the cultural first day of the week, and neither does this page.
Practical uses
Contracts and notice periods are usually counted in calendar days from a fixed date, and the relative-date tool states both the answer and what it counted from so the arithmetic can be checked rather than trusted. Recurring commitments — "the second Tuesday of the month", "the last Friday of the quarter" — are exactly what the ordinal weekday figures answer. Planning around a birthday or an anniversary is what the across-years table is for, including the February 29 case, where the honest answer for most years is that there is no such date.
For genealogy and archival research the historical mode matters more than the arithmetic: a parish register entry dated 3 September 1752 in England is either a transcription error or a date from a different reckoning, and knowing that the label never existed there is more useful than a weekday would be. And in data work, the ISO week figures are the ones that reconcile with reporting systems that group by week rather than by month.
Weekday
A weekday is a position in the unbroken seven-day cycle, not a property of a month or a year. Because the cycle has never been interrupted in the calendars modelled here, any two dates whose day counts differ by a multiple of seven share a weekday.
This page numbers weekdays the ISO 8601 way: Monday is 1 and Sunday is 7. That single numbering is used everywhere on the page, so a weekday number in the result card means the same thing as one in the calculation receipt.
Civil date
A civil date is what a calendar says: a year, a month and a day. It is not an instant in time. September 3, 2026 is one civil date everywhere on earth, even though the moment that date begins differs by up to a day between time zones.
Keeping the input a civil date rather than a timestamp is what makes the answer stable. There is no midnight to be pushed across a boundary, no offset to apply and no daylight-saving rule to trip over.
Gregorian calendar
The Gregorian calendar is today's international civil calendar. A year divisible by 4 is a leap year, unless it is also divisible by 100, unless it is also divisible by 400. That is why 1900 and 2100 are common years while 2000 and 2400 are leap years.
It was introduced in 1582 to stop the calendar drifting against the solar year, and it was adopted country by country over the following three and a half centuries.
Proleptic Gregorian calendar
“Proleptic” means the rules are projected backward beyond the date they were adopted. A proleptic Gregorian answer for a date in 1400 applies modern leap rules to a year in which nobody was using them.
This is the default convention here, and it is what most software means by a date in the distant past. It is a well-defined, reproducible convention — not a claim about what a calendar in 1400 actually said.
Julian calendar
The Julian calendar makes every fourth year a leap year, with no centurial exception at all. That is three leap days too many every four centuries, which is why it drifts against the Gregorian calendar by roughly one day per century.
It was the civil calendar of most of Europe for well over a millennium, and it is still used liturgically. A date before 1582 written in a contemporary document is usually a Julian date.
ISO week date
The ISO week date identifies a day as a week-year, a week number and a weekday: 2026-W36-4. Weeks start on Monday, and week 1 is the week containing 4 January — equivalently, the week holding that year's first Thursday.
A week belongs entirely to the year that holds its Thursday, so the ISO week-year can differ from the calendar year at either end of December and January. A year has either 52 or 53 ISO weeks.
Day of year
The day of the year counts from 1 on January 1 to 365, or 366 in a leap year. Because the count includes the date itself, the inclusive and exclusive readings differ by one, and mixing them up is a common off-by-one error.
This page states both: the day of the year counts the date, while “days elapsed before” and “days remaining after” both exclude it. The three always add back up to the length of the year.
Calendar reform gap
When a jurisdiction switched from the Julian to the Gregorian calendar, it deleted a run of civil date labels to absorb the accumulated drift. In Great Britain, Wednesday 2 September 1752 was followed by Thursday 14 September 1752.
The dates in between never existed there. They are not dates with an unknown weekday — they are labels nobody ever used, and this page reports them as such rather than inventing an answer. No time was lost: the two dates either side are consecutive physical days.
Gregorian, Julian and historically observed: which convention answers your question
Three different calculators can give three different weekdays for the same old date without any of them being broken. They are answering different questions. This is what each convention actually means.
| Aspect | Proleptic Gregorian | Julian | Historically observed |
|---|---|---|---|
| Leap-year rule | Divisible by 4, except centuries not divisible by 400 | Every year divisible by 4, no exception | Whichever rule was in force in the chosen jurisdiction on that date |
| Applies to dates before 1582 | Yes, by projecting modern rules backward | Yes, by projecting Julian rules backward | Yes, from a cited transition record |
| Needs a location | No | No | Yes — it is unavailable without one |
| 1900 is a leap year | No | Yes | Depends on the jurisdiction's calendar in 1900 |
| Can report a nonexistent date | Only for impossible dates such as April 31 | Only for impossible dates such as April 31 | Also for reform-gap labels such as 5 September 1752 in Britain |
| Best for | Software, data pipelines and any date after 1582 | Reading a pre-reform document written in Old Style | Asking what a calendar in one country actually said on a given day |
Verified calendar reform transitions
The historically observed mode answers only for jurisdictions in this dataset. Each record names the last date reckoned on the Julian calendar, the first date reckoned on the Gregorian one, and the civil date labels that were skipped and never used there.
| Jurisdiction | Last Old Style date | First New Style date | Skipped labels | Days omitted |
|---|---|---|---|---|
| Papal states, Spain, Portugal and Poland–Lithuania | Thursday 4 October 1582 (Julian) | Friday 15 October 1582 (Gregorian) | 5–14 October 1582 | 10 |
| Great Britain and the British colonies | Wednesday 2 September 1752 (Julian) | Thursday 14 September 1752 (Gregorian) | 3–13 September 1752 | 11 |
Great Britain omitted eleven days rather than ten because by 1752 the Julian calendar had drifted one further day behind: 1700 was a Julian leap year and not a Gregorian one. In both cases the two boundary dates are consecutive physical days — the labels jumped, the days did not.
Great Britain: Calendar (New Style) Act 1750 (24 Geo. 2 c. 23), legislation.gov.uk, consulted 2026-09-03. 1582 reform: NASA Goddard Space Flight Center, Calendars, consulted 2026-09-03.
Frequently asked questions
What day of the week is it today?
Open the Today tab. The page reads your device's local calendar date when it loads and reports the weekday, the day of the year, days remaining, the ISO week and whether it is a weekend. It is read in your browser, never on our server, which is why this one figure — and only this one — can differ between two readers in different time zones.
What is today's date?
The Today tab shows it in full, together with the ISO 8601 form (YYYY-MM-DD) and the ISO week date. It also names the time zone your browser reported, so you can see which local calendar the date came from.
How do I find the weekday for any date?
Use the Any Date tab and enter the month, day and year in three separate controls. The result gives the weekday plus the surrounding calendar context: day of the year, ISO week, the ordinal occurrence of that weekday in its month and year, and the full month laid out around your date.
What day of the week was I born on?
Enter your date of birth in the Any Date tab. Alongside the weekday you get the day of the year you were born and which occurrence of that weekday it was. The Same Date Across Years tab then shows what weekday your birthday falls on in each nearby year. Nothing you type leaves your browser.
What day will my birthday fall on this year?
Put your birth month and day into the Same Date Across Years tab and set the centre year to this year. You get a row per year with the weekday and whether it is a leap year, so you can see the whole run at once instead of checking one year at a time.
What week of the year is it?
Every result shows the ISO 8601 week number and the ISO week-year. Be careful with the two: an ISO week belongs to the year that contains its Thursday, so in late December or early January the ISO week-year can differ from the calendar year. The result says so explicitly when that happens.
What day of the year is today?
The Today tab and every Any Date result show it. January 1 is day 1, so the count includes the date itself. The page also reports days elapsed before the date and days remaining after it, both excluding the date, and those three figures always add back up to the length of the year.
What is the first day of the week?
It depends what you are asking. For ISO 8601 week numbering — used in this calculator's week dates — the week starts on Monday, and Monday is weekday 1. As a cultural and locale convention it varies: much of the world starts the week on Monday, the United States and several other countries on Sunday. You can switch the month calendar's display between the two; doing so never changes a calculated weekday or ISO week number.
How is the day of the week calculated?
By converting the civil date to a continuous count of days and reducing that count modulo seven. This page uses an integer day index in which 0001-01-01 is day 1 and a Monday, so the weekday is simply the remainder after dividing by seven. Everything else on the page — ISO week, day of year, ordinals — is arithmetic on the same integer.
What is Zeller's congruence?
A closed-form formula, published by Christian Zeller in the 1880s, that returns the weekday of a date directly from its year, month and day using integer division. It has separate Gregorian and Julian variants. It is one of several correct methods, not a standard anyone is required to use; this page's test suite uses it as an independent check on Julian results rather than as the production algorithm.
What is the Doomsday rule?
A mental-arithmetic method devised by John Conway. Certain dates in each year — 4/4, 6/6, 8/8, 10/10, 12/12 and others — all fall on the same weekday, the year's "doomsday", and you count from the nearest one. It is built for doing in your head, and it gives the same answer as any other correct method; it is a different route to the same weekday, not a different calendar.
Does my time zone affect a date I type in?
No. A date you enter is a civil date — a year, a month and a day with no time attached — so there is no instant for a time zone to shift. The engine never touches a clock, an offset or a daylight-saving rule. The only figure on this page that depends on where you are is today's date, because its input is your own device's calendar.
What is the difference between the Gregorian and Julian calendars?
The leap-year rule. The Julian calendar makes every fourth year a leap year with no exception. The Gregorian calendar removes three of those leap days every four centuries by skipping century years not divisible by 400, so 1900 and 2100 are common years while 2000 and 2400 are leap years. The Julian rule overshoots the solar year slightly, which is why the two calendars drift apart by roughly a day a century.
What does "proleptic Gregorian" mean?
It means today's Gregorian rules applied to dates before the calendar existed. A proleptic Gregorian answer for a date in 1400 is well defined and reproducible, but it is a convention, not a historical record: nobody in 1400 was using those rules. It is the default here, and it is what most software means by a date in the distant past.
Why do different calculators give different weekdays for old dates?
Usually because they are silently using different conventions. A proleptic Gregorian answer, a Julian answer and a historically observed answer for the same pre-reform date can all differ, and all three can be correct answers to different questions. The fix is labelling, not arithmetic: this page always states which convention produced the answer, and lets you see all three side by side.
What happened to the dates skipped during the calendar reform?
They were never used. When Great Britain adopted the Gregorian calendar, the Act of Parliament declared that the day after Wednesday 2 September 1752 would be called Thursday 14 September 1752, omitting eleven nominal days. Ask this page for one of those labels under the British jurisdiction and it tells you the date did not exist there, rather than inventing a weekday. No time was lost — the two dates either side are consecutive physical days.
How do I find the date of next Friday?
Use the weekday finder in the Today tab: pick Next and Friday, from today or from a date you selected. "Next" here means strictly after the reference date, so asking for next Friday from a Friday gives you the following week, not the same day. The inclusive reading — "this Friday", which can be today — is shown separately, because treating the two as synonyms is how off-by-seven-day mistakes happen.
How do I work out the date 10 days from today?
Use the relative date calculator in the Today tab: set the amount, choose days or weeks, choose before or after, and pick whether to count from today or from the date you entered. The result names both the answer and the date it counted from, so the arithmetic is checkable rather than something you have to trust.
References & authoritative sources
- Parliament of Great Britain via legislation.gov.uk (The National Archives) — Calendar (New Style) Act 1750 (24 Geo. 2 c. 23) · consulted September 3, 2026 · document date 1751 (in force 1752) · The primary instrument for the British transition. Enacts that the day after 2 September 1752 be called 14 September 1752, omitting eleven nominal days.
- NASA Goddard Space Flight Center — Calendars — Calendar Dates (Eclipse Web Site, adapted from the Explanatory Supplement to the Astronomical Almanac) · consulted September 3, 2026 · document date Continuously maintained · States the 1582 reform interval: ten days deleted, 4 October 1582 followed by 15 October 1582.
- U.S. Naval Observatory, Astronomical Applications Department — Calendars — Astronomical Information Center · consulted September 3, 2026 · document date Continuously maintained · Government reference on the Julian and Gregorian calendars and the 1582 introduction of the Gregorian reform.
- Edward M. Reingold and Nachum Dershowitz (Cambridge University Press) — Calendrical Calculations: The Ultimate Edition — authors' reference page for the book, papers and code · consulted September 3, 2026 · document date 2018 edition · The standard academic treatment of calendar arithmetic, and the origin of the fixed-day (Rata Die) representation this engine uses. Covers the Gregorian, Julian and ISO calendars.
- International Organization for Standardization — ISO 8601-1:2019 — Date and time: Representations for information interchange, Part 1: Basic rules · consulted September 3, 2026 · document date 2019 · The standard that defines the week date, Monday as weekday 1, and week 1 as the week containing 4 January. The publisher's catalogue page is linked; the standard text itself is sold by ISO and is not freely readable.
- Internet Engineering Task Force (IETF) — RFC 3339 — Date and Time on the Internet: Timestamps (Klyne and Newman) · consulted September 3, 2026 · document date 2002-07 · A freely readable profile of ISO 8601 for the YYYY-MM-DD representation this page accepts and emits.
- Unicode Consortium — UTS #35: Unicode Locale Data Markup Language (LDML) Part 4: Dates — week data and firstDay · consulted September 3, 2026 · document date Continuously maintained · The reference data for which day locales treat as the first day of the week — the evidence that calendar display start-of-week is a locale convention, separate from the ISO weekday numbering used in calculations.
Methodology, verification and sources
One dedicated engine (day-of-week-engine v1.0.0) runs every mode on this page. A date is held as a civil date — a plain year, month and day, with no time, no timestamp and no time zone — and is validated against the real length of its month on the calendar it claims to be on before any arithmetic runs. February 30, April 31, month 13 and February 29 in a non-leap year are rejected with a structured error rather than silently moved to a neighbouring real date, which is what JavaScript's own Date object would do. The validated date is then converted once to an integer day index: a Rata Die day number in which day 1 is 0001-01-01 on the proleptic Gregorian calendar, a Monday. Every figure the page shows — the weekday, the ISO week date, the day of the year, the ordinal occurrence of that weekday in its month and in its year, the relative dates and the neighbouring days — is integer arithmetic on that single number, so no two values on the page can come from parallel formulas and disagree. The Gregorian and Julian leap rules are implemented separately, over separate month tables, so neither can inherit the other's centurial exception. The historically observed calendar is not a rule in code: it resolves from the verified calendar-reform-transitions dataset, each record citing a primary instrument or a government scientific body, and it is unavailable without an explicit jurisdiction. Correctness is checked two ways on every build: frozen golden vectors, and a randomised comparison of ten thousand dates per calendar against a separately implemented weekday algorithm that shares no code with production.
- Validation: every date is checked against the real length of its month and year, on the calendar it claims to be on, before any arithmetic runs. An invalid date produces a structured error, never a silently corrected one.
- One integer representation: a single Rata Die day index underlies every figure on the page, so no two numbers here come from parallel formulas that could disagree.
- Separate leap rules: the Gregorian and Julian rules are implemented independently, over separate month tables, so the centurial exception cannot leak from one calendar into the other.
- Independent verification: the build compares the engine against separately implemented weekday algorithms — Sakamoto's for Gregorian, Zeller's congruence for Julian — over ten thousand deterministic pseudo-random dates each, spanning years 1 to 9999, and requires zero mismatches. Frozen golden vectors are additionally checked against an outside date library.
- Sourced history: the historical mode is driven by the calendar-reform-transitions dataset, verified 2026-09-03. Each record cites a primary instrument or a government scientific body, and the build asserts that every transition is continuous at the physical-day level.
- No time zone: only civil dates are used in any calculation. The single exception is reading your device's own local calendar date for the Today tab.
- Privacy: every calculation runs in your browser and the dates you enter are never sent anywhere. A share link carries only the inputs, in the URL fragment that browsers never transmit to a server, with no name or other identity field of any kind.
Assumptions
- Dates you enter are read on the proleptic Gregorian calendar by default: modern Gregorian leap rules extended backward and forward across the whole supported range of years 0001 to 9999. A date before the 1582 reform is read on that projected calendar, not on the calendar anyone alive at the time was using, unless you switch to the historically observed mode.
- A date is a civil date — a year, a month and a day. It is not a timestamp, and it carries no time of day, no UTC offset and no daylight-saving rule.
- Because an entered date has no time attached, a time zone cannot change its weekday. The same date resolves identically in Los Angeles, Rome, Tokyo and Kiritimati.
- Today's date is read from your browser's local civil date when the page loads, so it is the date on your own device's calendar. It is the only figure on this page that can differ between two readers.
- ISO week dates follow ISO 8601: Monday is weekday 1, and week 1 is the week containing 4 January, equivalently the week holding the year's first Thursday.
- The historically observed calendar is offered only for jurisdictions in the verified calendar-reform-transitions dataset, each backed by a cited primary or government source.
- Where a historical location is not specified, this page never guesses one. It answers on a named convention you chose, and says which one it used.
- The seven-day week is assumed to have run unbroken through the calendar reforms modelled here. That is what the record shows: the civil date labels jumped, but no weekday was skipped or repeated.
- Saturday and Sunday are treated as the weekend. That is a display convention, not a claim about any particular country, contract or industry.
Limitations
- There is no automatic historically observed calendar. Which calendar was in force on a date depends on where you mean, so the mode stays unavailable until you choose a jurisdiction.
- The historical transition dataset is deliberately limited to two jurisdictions — the 1582 papal reform and the 1752 British one — because those are the transitions whose skipped intervals are stated by a citable primary or government document. A jurisdiction that is not recorded cannot be selected.
- Years before 1 CE are not supported in this version. The range is 0001 to 9999; anything outside it is rejected rather than approximated.
- This page does no astronomical calculation. It reports civil calendar facts, not instants, sunrise, equinoxes or Julian Dates in the astronomical sense.
- Only the Gregorian and Julian calendars are implemented. Hebrew, Islamic, Persian, Chinese and other calendars are out of scope, and no conversion to them is offered.
- The first day of the week is a cultural and locale convention that varies by country. The calendar display here can start on Monday or Sunday, but that choice is presentation only and never changes a calculated weekday or ISO week number.
- Public holidays, business days and trading days do not follow from the calendar alone, and this page knows nothing about them.
- The Old Style / New Style change to the start of the legal year — from 25 March to 1 January in Britain — is a separate reform from the Julian-to-Gregorian day-count change, and is deliberately not applied to any date here.
Author: Ugo Candido · Editorial review: Ugo Candido, Founder & Editor-in-Chief () · Engine: CalcDomain calendar engine v1.0.0 · Share format: v1
Technical review: Dedicated day-of-week engine, share-state and renderer test suites run on every build: frozen golden vectors verified against an outside date library, 10,000 randomised Gregorian dates cross-checked against Sakamoto's algorithm and 10,000 randomised Julian dates against Zeller's congruence (neither shares code with production), physical-day continuity assertions on every calendar-transition record, and time-zone invariance checked by re-running the engine under five zones.
External professional review: Internal editorial and technical review completed 2026-09-03 by Ugo Candido, covering the calendar arithmetic, the ISO 8601 week rules, the cited transition records and the wording of the historical surfaces. No external professional audit has been commissioned for this page. See the public changelog.
Corrections: If a figure or a cited calendar-transition record looks wrong, write to the editor through the contact page with the date and the jurisdiction. Calendar arithmetic is checkable against a primary source, so a correction is either demonstrable or it is not; a demonstrated one is fixed in the engine, covered by a regression test, and recorded in the version history below and in the public changelog.
Version history
- · 1.0.0 — First publication. Dedicated day-of-week engine with an integer Rata Die day index, proleptic Gregorian and Julian conventions, ISO week dates derived from integer arithmetic, and a historically observed mode driven by the verified calendar-reform-transitions dataset.
- · 1.0.0 — Added the four-mode workspace (Any Date, Today and relative dates, Same date across years, Historical calendar), the month calendar, the calculation receipt and the versioned fragment-only share link.