Date calculator
Add or subtract days, months and years.
How it works
This calculator walks the calendar. To add three months, it advances the month value by three and lets the year roll over if needed. To add days, it steps day-by-day, respecting month lengths and leap years. Because months have different lengths, the answer to 'what is one month from 31 Jan?' can look surprising: the calculator gives you 28 Feb (29 in a leap year) rather than skipping to a non-existent 31 Feb. All arithmetic is local to your timezone. There's no server round-trip.
Frequently asked
How does it handle month-length differences?open
Adding one month to 31 January lands on 28 February in most years (29 in leap years) — the standard JavaScript rollover. It won't invent a 30 February.
Can I use negative numbers?open
Yes — either choose the Subtract mode, or enter a negative in Add mode. Both work.
Does the result change with timezones?open
No. The starting date is treated as midnight in your local timezone, so day-arithmetic is stable.