How the Date Calculator Works
What is the date 90 days from now? When is a payment due, if it must be paid 30 days after an invoice? What was the date 6 months ago? The Date Calculator answers these questions. It takes a start date, and adds or takes away years, months, weeks and days, or business days, and gives you the new date and the day of the week.
What you choose
- The start date. It is filled with today's date when you open the page.
- Add or subtract. Add gives a date in the future, and subtract gives a date in the past.
- What kind of amount. Years, months, weeks and days (you can use all four at the same time), or business days.
How years, months, weeks and days are added
- The years and the months are added first, together, as months (1 year = 12 months). The result has the same day of the month.
- If that month is shorter, so the day does not exist, the result is the last day of that month. So 31 January plus 1 month is 28 February (29 in a leap year), and 29 February 2024 plus 1 year is 28 February 2025.
- Then the weeks (7 days each) and the days are added to that date.
The rule is the same when you subtract. This is the way that calendars and most software count months. Note that "one month" is not always the same number of days, as the chart shows. Each block is about 0.8 of a day.
Jan ████████████████████████████████████████ 31 days Feb ████████████████████████████████████ 28 days Mar ████████████████████████████████████████ 31 days Apr ███████████████████████████████████████ 30 days May ████████████████████████████████████████ 31 days Jun ███████████████████████████████████████ 30 days Jul ████████████████████████████████████████ 31 days Aug ████████████████████████████████████████ 31 days Sep ███████████████████████████████████████ 30 days Oct ████████████████████████████████████████ 31 days Nov ███████████████████████████████████████ 30 days Dec ████████████████████████████████████████ 31 days
These are the days from the first of each month of 2026 to the first of the next month. Green months have 31 days, amber months have 30, and February has 28.
Worked examples
Start date: Friday, September 25, 2026.
| Add | New date |
|---|---|
| 7 days | Friday, October 2, 2026 |
| 30 days | Sunday, October 25, 2026 |
| 90 days | Thursday, December 24, 2026 |
| 1 month | Sunday, October 25, 2026 |
| 6 months | Thursday, March 25, 2027 |
| 1 year | Saturday, September 25, 2027 |
| 30 business days | Friday, November 6, 2026 |
Note that 30 days and 1 month are not the same: 30 days from 25 September is 25 October, but from 31 January 30 days is 2 March, and 1 month is 28 February.
Business days
If you choose business days, the calculator moves one day at a time and counts only the days from Monday to Friday. Saturdays and Sundays are skipped. For example:
- 1 business day after a Friday is the Monday after.
- 1 business day after a Saturday or a Sunday is also the Monday.
- 5 business days after a Monday is the next Monday, 7 calendar days later.
The more business days you add, the more calendar days that pass. Starting on a Monday, each block is about 2 calendar days:
1 business █ 1 calendar day 5 business ███ 7 calendar days 10 business ███████ 14 calendar days 20 business █████████████ 28 calendar days 30 business ████████████████████ 42 calendar days 60 business ████████████████████████████████████████ 84 calendar days
30 business days from Friday, September 25, 2026 is Friday, November 6, 2026. It is 42 calendar days, while 30 calendar days would end on Sunday, October 25, 2026.
Public holidays are not taken off, because they are different in each country, and often in each region. If your deadline skips holidays, add one day for each of them that falls in the period.
The two dates on a line
The calculator draws a line with the start date (blue) and the new date (green), and writes the number of calendar days between them. The day of the year of the result is also shown, for example 298 of 365.
Why the time zone does not matter
The calculation uses whole calendar dates, and not moments in time. This means that summer time (when a day has 23 or 25 hours) and the time zone of your computer never move the answer by a day.
Tips
- Deadlines: read the rule that applies. Some deadlines count the start day as day 1, and some count from the next day. This calculator counts from the day after the start.
- Due dates and pregnancy: a due date is often counted as 280 days from the first day of the last period. Add 280 days to find it. Your doctor gives the official date.
- Subscriptions and trials: add the number of days of the trial to the day you signed up.
- Long periods: the calculator works from the year 1 to the year 9999.
What this calculator does not include
- Public holidays, and weeks with a different number of working days.
- Hours and minutes. It works with whole days.
- Other calendars.
This tool is for general information. For a legal or contract deadline, check the rules that apply to you for how the days are counted.