Years Between Dates Calculator

A calendar year has 365 or 366 days, so the total-day count includes each actual February 29 in the range. The decimal-year display divides by the fixed 365.25-day Julian year. The calendar years-months-days summary uses a separate borrowing convention and can differ near month ends.

Choose the year definition that matches the task. A Julian year is exactly 365.25 days and is used as a scientific time unit. A Gregorian calendar year has 365 or 366 days and averages 365.2425 days across its 400-year cycle. A tropical year is an astronomical interval near 365.2422 days.

Birthdays advance on the calendar anniversary, not after 365.25 elapsed days. This calculator treats February 28 as the anniversary of a February 29 start date in non-leap years. Legal anniversary rules can vary by jurisdiction. Total days divided by 365.25 gives a separate decimal-year measurement for averaging or scientific work.

A Gregorian year is a leap year if it is divisible by 4, except if it is divisible by 100, unless it is also divisible by 400. So 2024 is a leap year (divisible by 4), 1900 is not (divisible by 100 but not 400), and 2000 is (divisible by 400). The rule keeps the calendar aligned with the tropical year of 365.2422 days. Skipping three of every four century leaps trims the calendar by exactly the right amount over a 400-year cycle. Pope Gregory XIII introduced it in 1582 to fix a 10-day drift the older Julian calendar had built up.

A date-only string can be interpreted as UTC by JavaScript and display as the previous date in a negative offset. Daylight saving transitions can also make local-midnight intervals 23 or 25 hours. This calculator parses local calendar dates and rounds elapsed milliseconds to whole days. It measures calendar-date separation, not wall-clock duration.