"Come back in 90 days." "Due 6 months from today." "Warranty expires 2 years after purchase." Adding a time period to a date sounds easy until you try it across a month boundary, a year change, or a leap day. This guide covers the exact rules and edge cases you need to get the answer right every time.
Adding days: the simplest case
Adding days only requires knowing the length of each month. January has 31 days, February has 28 (29 in leap years), March has 31, and so on. To add 10 days to March 25, count: 26, 27, 28, 29, 30, 31, April 1, 2, 3, 4. Answer: April 4.
For larger numbers, do it in chunks: add whole months of days first, then count the remainder. 100 days from June 10: June has 30 days → 20 days of June remaining → July (31) → August (31). So 100 − 20 − 31 − 31 = 18 days into September: September 18.
Adding weeks: easiest of all
Adding weeks is just adding multiples of 7 days — and the day of the week stays the same. 6 weeks from a Tuesday is still a Tuesday, 42 days later. "10 weeks from today" is trivial once you know "today" is a Tuesday.
Adding months: where it gets tricky
This is where most people get stuck. What is "one month after January 31"? Common answers: February 28, February 29 (leap year), March 3, March 31. All have some logical argument.
The most widely used convention, and the one our calculator uses, is:
- Bump the month forward by the requested number of months.
- If the target month does not have the same day number, use the last day of the target month.
So one month after January 31 is February 28 (or February 29 in a leap year). One month after March 31 is April 30. One month after May 31 is June 30. The day "overflows" to the month's last day.
Other conventions exist. Some systems add 30 days instead ("one month" = 30 days). Financial contracts often specify exactly which rule applies. Read the fine print.
Adding years: leap day trouble
Adding years is usually simple — same month, same day, year + N. March 5, 2024 + 5 years = March 5, 2029. But the edge case: what is one year after February 29, 2024? Most systems return February 28, 2025, since 2025 has no February 29. Add another year and you get February 28, 2026. Add to 2028 (leap year) and some systems "restore" the 29th — others do not. Be explicit in contracts and code.
Subtracting: same rules, in reverse
"90 days ago" uses the same logic as "90 days from now," just counting backward. If today is April 15, 90 days ago is: April (15) + March (31) + February (28 or 29) = 74 or 75 days. Still 15 or 16 days to subtract from the start of the year. 90 − 75 = 15 days. So roughly January 16.
For months and years, reverse the direction. "6 months before March 15, 2026" = September 15, 2025.
Mixing units: days + months + years
If you need to add "1 year, 3 months, 10 days," apply in order: first years, then months, then days. Order matters when overflows are involved — reverse order can land on a different date.
Example: Start date February 15, 2023. Add 1 year → February 15, 2024. Add 3 months → May 15, 2024. Add 10 days → May 25, 2024.
Practical scenarios
Due dates: Pregnancy due date is typically 280 days (or 40 weeks) from the last menstrual period. A project due "90 days from today" is 3 calendar months, give or take a day.
Warranty periods: "1-year warranty from purchase" almost always means to the same calendar date the next year — so a January 15, 2026 purchase is warranted through January 15, 2027.
Notice periods: A "30-day notice" in a lease usually means 30 calendar days. Some jurisdictions require it to be 30 days before the next rent payment date — an important nuance.
Tax filing: US tax extensions push due date forward by exactly 6 months. April 15 becomes October 15.
Common mistakes
Double-counting the start day. "1 day from Monday" is Tuesday, not Monday. The start date is day zero.
Ignoring leap years. "365 days from March 1, 2024" is February 28, 2025, not March 1, 2025, because 2024 has a Feb 29.
Assuming month = 30 days. For casual use that works. For contracts, specify calendar months explicitly.
Let the calculator do it
Our date add calculator handles every edge case — leap years, month overflows, mixed units — with a single click. Enter a start date, choose days, weeks, months, or years to add or subtract, and get the exact result. It is the fastest way to compute any future or past date without reaching for a calendar or doing the math in your head.