Contents
Add or subtract a calendar interval
Choose a date, an operation, and a number of days, weeks, months, or years. The calculator returns the new date, its weekday, an ISO check, and the exact number of calendar days moved.
Months need a calendar rule
Days and weeks have fixed lengths here: one week moves the date by exactly seven calendar days. Months and years do not. They first select a target month, then preserve the original day when that day exists.
In this expression, is the starting day number and is the number of days in the target month. This constrained behavior agrees with the calendar arithmetic described for TC39 Temporal calendars.
The actual day shift is calculated after the new date is known.
Dates that expose the difference
End of a leap-year February. Starting at 31 January 2024 and adding one month gives 29 February 2024, a Thursday. The actual shift is 29 days because the requested calendar position does not exist in February.
Moving backward by a month. Starting at 31 March 2024 and subtracting one month also lands on 29 February 2024, a Thursday. This time the signed shift is −31 days.
Crossing New Year. Adding one day to 31 December 2024 gives Wednesday, 1 January 2025. The ISO check is 2025-01-01, and the shift is exactly +1 day.
A leap day plus one year. Adding one calendar year to 29 February 2024 gives Friday, 28 February 2025, not 1 March. The actual shift is +365 days.
What the answer does not schedule
The calculation uses the proleptic Gregorian calendar from year 1 through 9999. The leap-year rule is the familiar one: a year divisible by 4 is a leap year, except a century year must also be divisible by 400. The US Naval Observatory calendar introduction documents this rule.
A calendar date has no time zone or time of day. That is why daylight-saving changes cannot add or remove an hour from this result. The tool also does not know public holidays, weekends, billing terms, or legal deadline rules.
The supporting ISO date follows the year-month-day order standardized by ISO 8601. Use it to verify a date whose local written form could otherwise be ambiguous.
Questions about adding time to a date
These answers define the exact calendar choices made by this calculator.
Why does 31 January plus one month end on 28 or 29 February?
The calculator first moves to February, then uses that month's final valid day because February has no 31st.
Is one month treated as 30 days?
No. A month changes the calendar month and can produce an actual shift of 28, 29, 30, or 31 days.
Is one year always 365 days?
No. The actual shift can be 365 or 366 days, depending on the dates crossed and leap-year clamping.
Are weekends and holidays skipped?
No. Days and weeks mean ordinary calendar days. Use a working-days calculator when a work calendar matters.
Does daylight saving time affect the result?
No. The input is a calendar date without a clock time or time zone.
Can the result go before year 1 or after year 9999?
No. Both the starting date and resulting date must remain inside the supported range from year 1 through 9999.
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