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Date & Timedates

Add or Subtract Days Calculator

Add or subtract a period from a date. Months are added before days and clamp at month end, which is why adding one month to 31 January gives 28 February rather than 3 March.

Also called: add days to date, date plus months.

Direction
Result date
2026-04-16

2026-04-16, a Thursday. That is 105 days from 2026-01-01, or 15 weeks and 0 days.

Day of the week
Thursday
Total days moved
105
In weeks and days
15 weeks and 0 days
Business days in between
75
On month-end clamping
No clamping was needed.
How this is calculated
Method and background

How this is calculated

Years and months are calendar units and days are fixed, so they are applied in that order. Adding a month to 31 January lands on a date that does not exist, and the convention is to clamp to the last day of the target month rather than roll forward. Different systems disagree about this, so the page says which was applied. Business-day counting skips weekends entirely and is a different operation from adding calendar days.

the order matters: adding a month to 31 January clamps to 28 or 29 February before any days are added
d
The start date

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

three months and fifteen days on

Start date
2026-01-01
Direction
Add
Years
0
Months
3
Days
15
Count the days as business days
No

Result date2026-04-16

1 April plus 15 days, counted independently

Open this example

a month from the 31st clamps

Start date
2026-01-31
Direction
Add
Years
0
Months
1
Days
0
Count the days as business days
No

Result date2026-02-28

boundary: the convention this page states

Open this example

Method and limits

What it assumes

  • UTC midnight throughout, so daylight saving cannot shift a result.

What it deliberately does not model

  • Public holidays are not known, so business days means weekdays.
  • Clamping is one convention among several; some systems roll into the next month instead.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

What is one month after 31 January?
28 February, or 29 in a leap year. The target date does not exist, so it clamps to the month end. Rolling forward to 3 March is the other convention and this does not use it.