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AgeWise Calculator

Add or Subtract Days From a Date

Start from any date, move forwards or backwards by days, weeks, months or years, and see the date you land on and its weekday.

What does this calculator do?

This tool moves a date forwards or backwards by an amount you choose and tells you where you land. It answers questions such as “what is the date 90 days from today?”, “when does a six-week trial end?” or “what was the date 18 months before the contract started?”. The result shows the full date and its weekday, so you can see straight away whether a deadline falls on a weekend.

Days and weeks are fixed lengths, so they are simply counted one calendar day at a time, including any 29 February along the way. Months and years are different: they follow the calendar. Adding one month keeps the same day number in the next month, and if that month is too short the result is clamped to its last day.

How to use it

  1. Enter the start date.
  2. Choose Add or Subtract.
  3. Type a whole number and pick days, weeks, months or years.
  4. Press “Calculate Date” and read the resulting date and weekday.

The start date itself is not counted, which matches how most notice periods and delivery estimates are written. If a rule you follow says “including the first day”, enter one day fewer.

Worked examples

Example 1: 100 days from 11 October 2026

Adding 100 days to Sunday 11 October 2026 lands on Tuesday 19 January 2027. Going the other way, 90 days before 11 October 2026 is Monday 13 July 2026.

Example 2: adding a month at the end of January

31 January 2026 plus 1 month gives Saturday 28 February 2026, because February 2026 has 28 days. In 2028, a leap year, the same step gives Tuesday 29 February 2028. Adding 1 year to 29 February 2028 gives Wednesday 28 February 2029.

Days versus months: which to choose

If a rule is written in days (“within 30 days”), count days, because 30 days and one month are often not the same. From 1 February, 30 days reaches 3 March in a common year, while one month reaches 1 March. When an agreement speaks in months or years, use those units so the result follows the calendar the way the agreement intends.

Frequently asked questions

What happens when I add a month to 31 January?

There is no 31 February, so the result moves to the last day of February: 28 February, or 29 February in a leap year. The same clamping applies to any month that is too short.

Is the start date counted as day one?

No. Adding 1 day to Monday gives Tuesday. If your rule counts the start date as day one, enter one fewer day.

How do I subtract instead of add?

Choose "Subtract" in the direction menu. The same amount is then counted backwards from the start date.

Are weekends or holidays skipped?

No, every calendar day counts. To count only working days between two dates, use the Business Days calculator.

Related tools

To measure the gap between two dates you already know, use Date Difference.