Add or Subtract Days from a Date
Add or subtract days, weeks, months or business days from a date.
Use via API
Add or subtract years, months, weeks and days (or business days) to a date. Month ends are clamped: 31 January + 1 month = 28/29 February. Send JSON, get JSON back.
POST https://pithlab.com/api/v1/tools/date-calculator
curl https://pithlab.com/api/v1/tools/date-calculator \
-H "Authorization: Bearer $PITHLAB_KEY" \
-H "Content-Type: application/json" \
-d '{"start":"2026-01-31","operation":"add","months":1,"days":10}'About the Date Calculator
Add or subtract years, months, weeks and days from any date, or count business days only, and see the resulting date and its day of the week.
How to use the Date Calculator
- Pick a start date.
- Choose Add or Subtract, then enter any mix of years, months, weeks and days.
- Tick “Business days only” to skip Saturdays and Sundays when counting the weeks and days.
- The resulting date and its weekday appear instantly, ready to copy.
Examples
Input
2024-01-31 + 1 month
Output
Thursday, 29 February 2024
Input
2026-10-19 + 90 days
Output
Sunday, 17 January 2027
Input
2026-12-24 + 10 business days
Output
Thursday, 7 January 2027
Input
2025-03-31 − 1 month
Output
Friday, 28 February 2025
Frequently asked questions
What happens when I add a month to 31 January?
The date is clamped to the last day of the shorter month: 31 January + 1 month is 28 February, or 29 February in a leap year. The same applies when subtracting, so 31 March − 1 month is the end of February.
In what order are the years, months, weeks and days applied?
Years and months first, then weeks and days. 31 January + 1 month + 1 day is therefore 1 March in a common year (28 February plus a day), not 32 February.
How do business days work?
With “Business days only” ticked, weeks count as 5 working days and each day skips Saturdays and Sundays. Starting on a weekend, the first business day is the following Monday. Public holidays aren't skipped.
Does the start date count as day zero?
Yes. Adding 1 day to Monday gives Tuesday, and adding 0 days gives the start date itself.