Date Add and Subtract — a Start Date Plus a Duration Gives the Due Date
Add or take away years, months, weeks, days, hours and minutes from a date and see the result with its weekday, how far it sits from today, and how the month-end and leap-year rounding was handled.
One way or the other: typing a sentence such as 2mo 10d fills the six boxes underneath, and those boxes are what gets counted.
Order of addition and month-end pullback
One date, one duration, one direction
Contract end dates, notice periods, a due date, a warranty that runs 25 months, a shift that starts Friday at 22:00 and lasts 14 hours. The shape is always the same: a date, an amount, a direction — and the answer has to land on a real calendar day.
The order the amount is applied in
Years and months go first, because they are the only part that depends on which month you end up in. Weeks and days next, since they are plain calendar steps. Hours and minutes last, because they are the only part that can change the date.
That order is visible when the start is near the end of a month. 2026-01-31 plus 1 month plus 1 day is 2026-03-01: February has 28 days, so the month step pulls the date back to the 28th and the day step then walks forward one. Add the same thing as one 32-day step instead and you get 2026-03-04. Whenever a pull-back happens, the result line says so — the rounding is never silent.
Which month ends get pulled back
A month step keeps the day number when the target month has it and clamps to the last day when it does not. The three cases that come up in practice:
| Start | Add | Result | Why |
|---|---|---|---|
| 2026-01-31 | 1 month | 2026-02-28 | 2026 is not a leap year |
| 2024-01-31 | 1 month | 2024-02-29 | 2024 is a leap year |
| 2026-01-30 | 1 month | 2026-02-28 | 30 February does not exist |
The clamp does not carry the leftover days into the next month, which is what payroll, rent and interest schedules normally expect.
Backwards works the same way
Subtracting months from the 31st of a long month lands on the last day of the short month, and the note appears just the same. The two direction buttons only choose the sign; nothing else changes.
Reading the amount: the sentence box accepts numbers with units — 2y, 18mo, 3w, 45d, 6h, 90min, and mixtures such as 1mo 2w 3d. A bare m means months, so write minutes as min. Fractions are not read; write 1 week 3 days rather than 1.5 weeks. Typing a sentence fills the six boxes below it, and those boxes are what gets counted, so you can always see the exact amount behind the answer.
Questions
Does 29 February exist in the result? The date and the weekday are both printed, so you can see it. The table above covers the only case where a day disappears.
Do the dates leave the device? No. Every figure comes from the script and the browser’s own calendar, and what you type never leaves this device.
Want the other direction? Given two dates and asked how far apart they are, that is the days between two dates page.