Date Calculator
Add or subtract days from a date, find the duration between two dates, and look up the day of the week for any date.
Runs entirely in your browser. Nothing is sent to our servers.
About this tool
Three date utilities in one:
- Add / subtract — add days, months, or years to a date. Useful for finding deadline dates, expiry dates, or renewal dates.
- Duration between — find the gap between two dates in days, weeks, months, and years. Useful for measuring projects, anniversaries, or "how long has it been?"
- Day of week — find what day of the week any date falls on. Useful for "what day was I born?" or "what day will Christmas be in 2028?".
Calendar caveats
Months aren't all the same length, so "add 1 month" can be ambiguous — January 31 + 1 month is February 28 (or 29 in leap years), not "February 31". This tool follows the standard convention: snap to the last day of the target month when the day-of-month doesn't exist there.
All dates use the proleptic Gregorian calendar, so the math is consistent even for very old dates. Most countries didn't adopt the Gregorian calendar until the 1600s–1900s, so historical dates may differ from what was recorded at the time.
Frequently asked questions
- Why does adding 1 month to January 31 give February 28?
- Because there's no February 31. The standard convention is to snap to the last valid day of the target month. Different systems differ — some go to March 3 instead — so we picked the most common one.
- How is "months between" calculated?
- As whole months — from one same-day to the next. Jan 15 to Feb 15 is 1 month, Jan 15 to Feb 14 is 0 months plus 30 days. This matches how people usually mean "5 months" in casual speech.
- What about business days only?
- For business-day counting between two dates (excluding weekends and holidays), use the Statutory Holiday Calculator — it has a business-day counter that respects each province's holidays.
- Does the day-of-week calculator work for dates before 1583?
- It uses the proleptic Gregorian calendar — what the modern calendar would have said for those dates if it had existed. Actual historical records used the Julian calendar before various dates of Gregorian adoption (1582 onwards by country).
Last updated: May 17, 2026