Date Calculator: Add or Subtract from a Date
Pick a date, enter a number, choose days, months, or years. Adding 1 month to January 31 gives February 28 (or February 29 in a leap year) — the result always lands on a real date.
- Day of the week
- Days from start
How the math works
Days are exact: adding 10 days always moves the calendar forward by exactly 10 days, across month and year boundaries. Months and years are calendar units: adding 1 month keeps the same day number when possible, and clamps to the end of shorter months when it is not. January 31 plus 1 month is February 28; March 31 minus 1 month is February 28 (or 29 in a leap year).
Negative amounts subtract. Adding −90 days is identical to subtracting 90 days, which makes the single amount field useful for deadlines in both directions — "90 days from today" and "90 days before the launch".
Worked examples
- 2025-01-31 + 1 month → February 28, 2025 (Friday).
- 2024-01-31 + 1 month → February 29, 2024 (Thursday) — leap year.
- 2024-02-29 + 1 year → February 28, 2025 — leap days clamp.
- 2023-12-31 + 1 day → January 1, 2024 — year boundary, handled.
- 2026-10-17 − 100 days → July 9, 2026.
Frequently asked questions
What happens when I add a month to January 31?
The result clamps to the last real day of the target month: January 31 plus 1 month is February 28, or February 29 in a leap year. The calculator never invents dates like "February 31" — it always lands on a day that exists.
How do I subtract instead of add?
Enter a negative number. Adding −10 days is the same as subtracting 10 days. The amount field accepts any whole number, positive or negative.
What happens to February 29 birthdays when I add years?
February 29 plus 1 year becomes February 28 of the following year (since that year has no 29th). Add 4 years and you land back on February 29. This matches how most age and anniversary calculations treat leap days.
How far into the past or future can I calculate?
The calculator handles any Gregorian calendar date your browser can represent — centuries in either direction work fine, including year boundaries and leap years like 2000 and 2100.
Does adding "1 month" mean 30 days?
No. A month here is a calendar month, not a fixed number of days. Adding 1 month to March 15 gives April 15 (31 days later), while adding 1 month to April 15 gives May 15 (30 days later). Use the days unit when you need an exact day count.