A date calculator answers "what date will it be?" — or "what date was it?" — when you shift a start date by some amount of time. Enter a date, choose add or subtract, and give any mix of years, months, weeks and days. It applies them by the real calendar (so months and years respect their actual lengths and leap years), then reports the resulting date, its weekday, and the total days elapsed — exactly what you need for deadlines, due dates and notice periods.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: standard calendar arithmetic, recomputed in code.
How date arithmetic works
The calculator shifts the year first, then the month, then the days (weeks are just seven days each). Years and months follow the calendar rather than fixed blocks: adding a month to 15 January gives 15 February, not "30 days later." If the start day doesn't exist in the target month — say you add a month to 31 January — the date rolls forward into the next month, which is the standard convention. Leap years are handled automatically because real dates are used throughout.
Worked example — 15 Jan 2026 + 3 months 10 days
Scenario: start 15 January 2026, add 3 months and 10 days.
The result is Saturday, 25 April 2026, exactly 100 days after the start. Notice how mixing units works: the three months move you across the calendar (respecting the 28-day February and 31-day March), and the ten days finish the job. The "days elapsed" figure collapses that mixed span into a single number — handy when a contract says "90 days" but you'd rather think in whole months.
Frequently Asked Questions
Enter the date, choose Add, type the days. 26 Jun 2026 + 90 days = Thu, 24 Sep 2026.
By the calendar. +1 year on 26 Jun 2026 = 26 Jun 2027; +3 months on 15 Jan = 15 Apr.
Yes — choose Subtract. 26 Jun 2026 − 30 days = Wed, 27 May 2026.
The total calendar days between start and result. 15 Jan → 25 Apr 2026 = 100 days.
Leap years handled automatically. Whole dates only — no time-zone adjustment.