"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.
Here is the same chunk method step by step, for 90 days from April 21, 2026:
- Days remaining in April: 30 − 21 = 9.
- May: 31 days.
- June: 30 days.
- Running total: 9 + 31 + 30 = 70 days (through the end of June).
- Remaining: 90 − 70 = 20 days into July.
- Answer: July 20, 2026.
Common offsets from one start date
Applying the same method to other common offsets from April 21, 2026:
| Offset | Landing date |
|---|---|
| 30 days | May 21, 2026 |
| 60 days | June 20, 2026 |
| 90 days | July 20, 2026 |
| 120 days | August 19, 2026 |
| 180 days | October 18, 2026 |
| 365 days | April 21, 2027 |
For a quick sanity check, these mental-math shortcuts are often good enough for casual planning:
- 30 days ≈ 1 month (exact in months with 30 days)
- 90 days ≈ 3 months
- 180 days ≈ 6 months
- 365 days = 1 year (unless the span crosses a leap day, then 366 = 1 year)
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.
Months vs. days: not the same thing
"3 months" and "90 days" sound interchangeable, but they don't always land on the same date. 3 calendar months from April 21 is July 21 (same day number, different month). 90 days from April 21 is July 20. The difference comes from month lengths: April, May, and June have 30, 31, and 30 days, which average 30.33.
That is why a contract that says "within 90 days" and one that says "within 3 months" can expire on different dates. Check which unit the document actually uses.
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. That leaves 16 or 15 days to subtract from the end of January. In a leap year, 90 − 75 = 15 days, landing on January 16; in other years, 90 − 74 = 16 days, landing on January 15.
For months and years, reverse the direction. "6 months before March 15, 2026" = September 15, 2025.
Counting backward is also a good way to check a forward calculation. 90 days before July 20: 20 days back to June 30, then 30 days of June to May 31, then 31 days of May to April 30, then 9 more days to April 21 — matching the forward example above.
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.
Calendar days vs. business days
"Due in 30 days" almost always means 30 calendar days. "Due in 30 business days" means about 42 calendar days (6 weeks). A "30-day notice" in a lease is normally calendar days; a "30-day" SLA in a contract might be either, so check the definition.
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 roughly 3 calendar months, give or take up to two days, because three calendar months span 89 to 92 days.
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.
Estimated taxes: IRS quarterly estimated tax payments are due at roughly 3-month intervals — April 15, June 15, September 15, and January 15 of the following year. Note the irregular gap: the second payment comes only 2 months after the first.
Payment terms: "Net 30" on an invoice issued March 15 means payment is due April 14, 30 days later.
Return windows: Most US retailers use 30, 60, or 90-day return windows from the purchase date. A December 1 purchase with a 90-day window can be returned through March 1 — or February 29 when the following February has a leap day.
Probation periods: A "90-day probation" at a new job means 90 calendar days from the start date. A January 2 start ends April 2 (April 1 in a leap year).
Cancellation notice: A "60-day cancellation notice" means you must give notice 60 days before the cancellation takes effect — so work backward from the target date.
Security deposit returns: State law varies. In California, deposits must be returned within 21 calendar days of move-out; in Texas, 30 days. Always verify your own state's rule.
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 February 1, 2024" is January 31, 2025, not February 1, 2025, because the span includes February 29, 2024. By contrast, 365 days from February 1, 2023 is exactly February 1, 2024 — no leap day falls in between.
Assuming month = 30 days. For casual use that works. For contracts, specify calendar months explicitly.
Expecting a 30-day offset to stay in the next month. January 31 + 30 days = March 2, because February has only 28 days in most years. It looks odd, but it is correct if you count day by day.
Forgetting about weekends. If a deadline lands on a Saturday, many organizations accept payment the following Monday. Check the specific rules before relying on it.
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.