About

An invalid date propagated through a financial system, a database, or an engineering log can corrupt records irreversibly. February 29 in a non-leap year, September 31, or any date falling within the Gregorian reform gap (October 5-14, 1582) are common sources of silent data corruption. This tool validates dates against the full proleptic Gregorian and Julian calendar rule sets. It computes the leap year status using the divisibility chain (y ÷ 4, exception at 100, counter-exception at 400 for Gregorian; simple y ÷ 4 for Julian), resolves the day of week via Zeller's congruence, and outputs the Julian Day Number for astronomical cross-referencing.

The validator rejects impossible dates outright and flags ambiguous historical dates near the 1582 reform boundary. It also returns the ISO 8601 week number, the day-of-year ordinal, and the quarter. Note: this tool uses the proleptic Gregorian calendar for dates before October 15, 1582, which is a mathematical extension and may not match historical records of regions that adopted the Gregorian reform later (e.g., Britain in 1752, Russia in 1918). Pro tip: always specify which calendar system your source data uses before storing historical dates.

Formulas

Gregorian leap year predicate:

L = (y mod 4 = 0) ∧ (y mod 100 ≠ 0 ∨ y mod 400 = 0)

Where y = year, L = TRUE if leap year.

Julian Day Number (Gregorian calendar):

JDN = 367y − floor(7(y + floor(m + 912))4) + floor(275m9) + d + 1721013.5

Where y = year, m = month, d = day.

Zeller's congruence for day of week (Gregorian):

h = (q + floor(13(m + 1)5) + K + floor(K4) + floor(J4) − 2J) mod 7

Where q = day of month, m = month (3 = March … 14 = February, with Jan/Feb counted as months 13/14 of the previous year), K = year of century (y mod 100), J = zero-based century (floor(y ÷ 100)), h = day of week (0 = Saturday … 6 = Friday).

ISO 8601 week number is computed by finding the Thursday of the target week and counting weeks from January 1 of that Thursday's year.

Reference Data

Month	Days (Common Year)	Days (Leap Year)	Quarter	Day-of-Year Start (Common)	Day-of-Year Start (Leap)
January	31	31	Q1	1	1
February	28	29	Q1	32	32
March	31	31	Q1	60	61
April	30	30	Q2	91	92
May	31	31	Q2	121	122
June	30	30	Q2	152	153
July	31	31	Q3	182	183
August	31	31	Q3	213	214
September	30	30	Q3	244	245
October	31	31	Q4	274	275
November	30	30	Q4	305	306
December	31	31	Q4	335	336
Gregorian Leap Year Rule
Divisible by 4			Leap year candidate
Divisible by 100			Not a leap year (exception)
Divisible by 400			Leap year (counter-exception)
Julian Leap Year Rule
Divisible by 4			Always a leap year
Notable Calendar Reform Dates
Gregorian reform (Papal)			Oct 15, 1582
Britain & colonies			Sep 14, 1752
Russia			Feb 14, 1918
Greece			Mar 1, 1923
Turkey			Jan 1, 1926
ISO 8601 Week Numbering
Week starts on			Monday
Week 1 contains			January 4
Weeks per year			52 or 53

Frequently Asked Questions

When Pope Gregory XIII reformed the calendar in 1582, the dates October 5 through October 14 were skipped entirely to correct the accumulated drift of the Julian calendar. These 10 dates never existed in the Gregorian system. The validator flags any date in this gap as invalid. If you are working with historical records from a region that adopted the reform later (e.g., Britain in 1752), switch to Julian mode for dates before that region's adoption date.

The Julian calendar uses a simple rule: any year divisible by 4 is a leap year. The Gregorian calendar adds two exceptions: years divisible by 100 are not leap years unless they are also divisible by 400. This means the year 1900 is a leap year in the Julian calendar but not in the Gregorian calendar. The year 2000 is a leap year in both systems because it satisfies the 400-year counter-exception.

The Julian Day Number (JDN) is a continuous count of days since the beginning of the Julian Period on January 1, 4713 BC (proleptic Julian calendar). Astronomers use it to avoid ambiguity between calendar systems and to simplify date arithmetic. For example, to find the number of days between two dates, subtract their JDNs. The JDN for January 1, 2000 (Gregorian) is 2451545.

Yes. Enter the year as a negative number or zero. Year 0 corresponds to 1 BCE in astronomical year numbering (ISO 8601). The proleptic Gregorian and Julian calendars extend backward indefinitely, so the leap year rules still apply. Note that no civilization used either calendar system before its invention, so these dates are mathematical constructs.

ISO 8601 defines week 1 as the week containing the first Thursday of the year (equivalently, the week containing January 4). Weeks start on Monday. This means December 29-31 may belong to week 1 of the following year, and January 1-3 may belong to week 52 or 53 of the previous year. The validator computes this correctly by finding the Thursday of the target date's week and determining which year that Thursday falls in.

No. Calendar date validity is purely arithmetic. A date is valid or invalid regardless of timezone or locale. However, the day-of-week result assumes the proleptic Gregorian or Julian calendar without timezone adjustments. If you need to validate a date-time with timezone awareness (e.g., does February 29 exist in UTC+14?), the date portion is still governed by the same leap year rules.