Add Days Calculator

day and week shifts are exact; a month or year shift keeps the day number unless the target month is too short, in which case it clamps to that month's last day

date

The starting point, which is never adjusted

amount

Whole days, weeks, months or years; a negative value moves the date backwards

unit

Days and weeks are fixed lengths, months and years are not

shiftDays

The real distance moved, which differs from the amount whenever months or years are involved

Use it to find a due date, a follow-up window, or the end of a subscription period. Days and weeks are exact, so a 14-day shift always lands on the same weekday and the distance moved always equals the amount. Months and years are not, so a one-month shift moves 28, 29, 30 or 31 days depending on where it starts — the days moved output tells you which of those happened. What it cannot do is follow a contract's own rule for period ends: if an agreement says a term ends on the last business day, that is a different calculation.

1. One month after 31 January 2026

   1. The target month is February 2026, which has 28 days — there is no 31st to land on
   2. The day therefore clamps to the last day of that month, giving 28 February 2026
   3. Days moved: 31 January to 28 February is 28 days, not 31
   4. The weekday follows the result: 28 February 2026 is a Saturday

   Clamping is the part of this page that is easiest to get wrong. A tool that implements one month as thirty days lands on 2 March instead.

2. One month after 31 January 2024

   1. 2024 is divisible by 4 and not by 100, so February has 29 days
   2. There is still no 31st to land on, so the result clamps to the last day again — this time the 29th
   3. Days moved is therefore 29, one more than the same shift in a common year
   4. The two examples together show that the clamp targets the last day of a month, not a fixed number of days

   Same starting date, same instruction, one day apart because of the year. That difference is only visible because the days moved is reported alongside the date.

3. 90 days after 1 January 2026

   1. A day shift does no clamping at all: 1 January plus 90 days is simply the date 90 days later
   2. January 2026 has 31 days, February has 28 and March has 31, and 31 + 28 + 31 = 90 — exactly three calendar months
   3. The result is therefore 1 April 2026, and the days moved is the 90 that was asked for
   4. 1 April 2026 is a Wednesday while 1 January was a Thursday, because 90 is not a multiple of seven

   Days and weeks are fixed lengths, so nothing here can surprise anyone. The two month examples above are where the surprises live.

Months and years are not fixed lengths, so one month after a date means landing on the same day number or, when the month is too short, on its last day. That clamping rule is a convention shared with spreadsheet software, and a tool that rolls over into the next month instead will answer one to three days later. Nothing here skips weekends or public holidays — the business days tool does that. Dates are proleptic Gregorian throughout, so no calendar reform is applied.

How do I add 90 days to a date?

Add 90 to the day number rather than touching the calendar fields. 1 January 2026 plus 90 days is 1 April 2026, because January has 31 days, February 28 and March 31 — those three months consume exactly 90 days. A week shift is the same with a factor of seven, and neither can clamp.

What is one month after 31 January?

28 February in a common year, and 29 February in a leap year. There is no 31 February, so the result clamps to the last day of the target month rather than rolling into March. This is the rule spreadsheet software uses, and it is why a one-month shift can move 28 days instead of 31.

Does adding a month keep the same day number?

It keeps it whenever the target month is long enough, so 15 March plus one month is 15 April. When the target month is shorter, the day clamps to that month's last day: 31 January plus one month is 28 February, and 31 March plus one month is 30 April. Shifts from the 29th, 30th and 31st are where the two rules part company.

Can I subtract days from a date instead?

Yes, enter a negative amount. Subtracting 10 days from 19 September 2026 gives 9 September 2026, and the days moved reads minus ten, so the direction shows up in the result rather than only in the input. Subtracting a month clamps exactly the way adding one does.

How many days is a one-month shift?

Between 28 and 31. The days moved output does the counting rather than leaving it to you: a one-month shift from 31 January is 28 days in 2026 and 29 days in 2024. Wherever the exact count matters — interest, notice periods, warranty terms — read that number and not the word month.

• relativedelta — the dateutil module — dateutil project documentation
• RFC 3339: Date and Time on the Internet: Timestamps — IETF