The three-part Gregorian rule
A Gregorian year is a leap year when its number is divisible by 4, except that a year divisible by 100 is common unless it is also divisible by 400. Leap years contain 366 days and add February 29 between February 28 and March 1. Common years contain 365. This correction keeps the civil calendar aligned more closely with the seasonal year than adding one day in every fourth year without exception.
The rule can be implemented as: divisible by 400, or divisible by 4 but not by 100. Test the century condition rather than stopping after divisibility by 4. The U.S. Naval Observatory and National Research Council Canada both document this same civil-calendar rule. It is a calendar fact, distinct from any policy defining when a person reaches a regulated age.
1900 was not a leap year
Birth date: February 28, 1899. Reference date: March 1, 1900. Counting rule: apply all Gregorian divisibility tests before counting elapsed days. Calculation: 1900 ÷ 4 is exact, but 1900 is divisible by 100 and not by 400, so February 1900 has 28 days; the interval totals 366 elapsed days. Result: calendar age is 1 year and 1 day. Interpretation: assuming a February 29, 1900 would add a nonexistent day.
The interval still has 366 days because it begins before March in 1899 and ends after February in 1900; the location of the annual endpoints matters. The leap classification alone does not tell an interval's length. Identify whether February 29 actually lies after the start and on or before the end.
2000 was a leap year
Birth date: February 28, 1999. Reference date: March 1, 2000. Counting rule: century years divisible by 400 regain leap status. Calculation: 2000 is divisible by 4, 100, and 400, so February contains day 29; February 28, 1999 to February 28, 2000 is 365 days, then February 28→29 and February 29→March 1 add 2. Result: 367 elapsed days and a calendar age of 1 year, 2 days. Interpretation: the 400-year exception preserves February 29, 2000.
This is why software that uses only year % 4 can appear correct for many modern dates yet fail at century boundaries. Tests should include at least one excluded century and one retained century, not only familiar years such as 2024.
2100 will not be a leap year
Birth date: February 28, 2099. Reference date: March 1, 2100. Counting rule: apply the century exception prospectively. Calculation: 2100 is divisible by 4 and 100 but not 400, so its February has 28 days; the date difference is 366 days. Result: calendar age is 1 year, 1 day. Interpretation: a four-year pattern inferred from 2096 and 2104 would be wrong at 2100.
Future calendar calculations can apply the Gregorian arithmetic rule, but they cannot predict future administrative policies. The tool treats entered dates using the same proleptic Gregorian rules supplied by the JavaScript date system. A scheduling or archival system should document its supported date range and calendar rather than assuming every historical culture used Gregorian dates.
A leap day changes total days, not the birthday count
Birth date: April 10, 2019. Reference date: April 10, 2025. Counting rule: calendar years count completed April 10 anniversaries; elapsed days count every intervening boundary. Calculation: six annual anniversaries have occurred, while February 29, 2020 and February 29, 2024 each add a day to the total; 4 common-year lengths plus 2 leap-year lengths = 2,192 days. Result: 6 years, 0 months, 0 days and 2,192 elapsed days. Interpretation: leap days enlarge the elapsed total but do not create an extra calendar year.
Saying someone gained an extra day of age in a leap year confuses a measurement total with the anniversary count. Everyone whose interval crosses that date accumulates the additional elapsed day. The completed-year value changes only at the selected annual anchor.
Intervals that cross February 29
To determine whether a leap day contributes, inspect the actual half-open interval from start date to reference date: the starting midnight is excluded and the ending midnight completes the final day. February 28 to March 1 in 2024 contains two elapsed days, while the same labels in 2023 contain one. February 29 to March 1 contains one day.
A multi-year interval can contain several leap days, but counting leap-year labels is not enough if the interval starts after February or ends before it. Direct UTC date subtraction accurately incorporates the actual dates. For a manual calculation, split the span at year boundaries and include February 29 only where it falls inside the endpoints.
Historical scope of the Gregorian calendar
The Gregorian calendar was introduced in 1582 and adopted at different times in different places. A modern software library may extend its rules backward before local adoption; that is a proleptic Gregorian calculation. It can be internally consistent without matching the calendar printed in a historical document. USNO material distinguishes Gregorian and Julian calendars and notes the historical changeover issue.
For genealogy or historical research, identify the source calendar and local adoption history before entering a date. NumUtility offers no Julian/Gregorian conversion and does not repair omitted historical dates. Its age calculation is best understood as arithmetic on entered Gregorian date labels, not universal historical chronology.
Scope, checks, and the separate leap-day birthday question
This guide explains leap years as calendar structure. The leap-day birthdays guide focuses on people recorded as born February 29, personal anniversary choices in common years, and variation in official rules. Keeping those questions separate prevents a mathematical divisibility rule from being presented as a legal anniversary rule.
Common mistakes include treating every fourth year as leap, treating 1900 as leap, treating 2000 as common, counting a leap day outside the interval, and converting calendar age through years × 365. Test 1900, 2000, and 2100 explicitly. The Age Calculator includes leap days in Total days, uses a stated March 1 common-year convention for February 29 calendar decomposition, rejects reversed age dates, and does not decide official eligibility or legal effective dates.
Testing a leap-aware implementation
A strong test set surrounds each boundary. For 2000, compare February 28, February 29, and March 1; for 1900 and 2100, confirm February 28 is followed directly by March 1. Include intervals starting before and after February because leap status affects a total only when the extra day lies inside. Also test equal dates, reversed age inputs, and January 31 month anchors.
Keep calendar-decomposition tests separate from total-day tests. The former must reconstruct the reference date under a declared convention and never return negative components. The latter must equal UTC-midnight subtraction. This separation exposes software that displays attractive calendar units while miscounting days, or vice versa, and makes century regressions visible long before 2100. Include 2024 so famous century cases cannot be hard-coded. Check days immediately around every anchor and ensure browser time-zone changes leave date-only totals unchanged. Historical tests should identify whether their labels are proleptic Gregorian.