Date Add/Subtract Calculator
Add or subtract days, weeks, months, or years from any date
Frequently Asked Questions
Adding or subtracting a number of days, weeks, months, or years from a starting date sounds like simple arithmetic, but months and years don't have a fixed, consistent length in days — which means "add one month" isn't the same operation as "add 30 days," and getting this distinction wrong produces a date that's off by a day or more.
Why calendar-unit math isn't the same as day-count math
Adding a fixed number of days is straightforward day-counting arithmetic, unaffected by calendar structure. Adding a month, however, means landing on the same day-of-month in the following calendar month — which could be 28, 29, 30, or 31 days later depending on which specific months are involved, not a fixed 30-day jump. Adding a year similarly means landing on the same month and day in the following year, adjusted for leap years when the date in question is February 29 (which doesn't exist in non-leap years, requiring a defined rule for how that edge case gets handled). Treating "add a month" as "add 30 days" produces a result that's subtly wrong for at least seven months of the year (the ones with 31 days) and wrong in the opposite direction for February.
A worked example
Adding one month to a date late in a 31-day month, versus adding 30 days to that same date, can land on two different calendar days — the calendar-aware "add one month" operation lands on the same day-of-month in the next month, while the naive 30-day addition drifts by a day or more depending on which months are actually involved.
How this connects to your other date calculations
For measuring the gap between two existing dates rather than projecting forward or backward from one date, the Date Difference Calculator handles that related but distinct calculation. For finding which day of the week a specific resulting date falls on, the Day of the Week Calculator answers that follow-up question directly.
Common mistakes
Approximating "add N months" as "add N×30 days" is the most common shortcut that produces a subtly wrong result — for any date calculation where landing on the exact correct calendar date matters (a legal deadline, a contract renewal date, a religious observance), calendar-aware month/year arithmetic gives the correct answer where a fixed-day approximation doesn't.