Date add / subtract

Finding the date 90 days after a contract starts, 6 months before a deadline or 72 hours after an event sounds simple, but varying month lengths, leap years and daylight saving time make it easy to slip by a day. This calculator adds or subtracts any combination of years, months, days, hours and minutes from a starting date and time.

How to use it

1. Pick the start date and time.
2. Enter the amounts to add or subtract in each unit.
3. Read the resulting date, including the weekday.

Examples

• 15 January 2026 + 90 days = 15 April 2026 (Wednesday).
• 27 October 2026 - 6 months = 27 April 2026.
• 31 January 2026 + 1 month = 28 February 2026, because February has no 31st.
• 20:00 on 30 March + 12 hours = 08:00 on 31 March.

Month arithmetic rules

Adding months keeps the day number when possible. If the target month is shorter, the result is clamped to its last day. That means adding one month and then subtracting one month does not always return to the original date: 31 January + 1 month = 28 February, and 28 February - 1 month = 28 January.

Days vs. months

Adding 30 days and adding 1 month give different results most of the time. Contracts, subscriptions and legal deadlines specify which one applies. Payment terms such as net 30 usually mean 30 calendar days, while a monthly subscription renews on the same day number each month.

Time zones and DST

Adding 24 hours across a daylight saving change can land at a different clock time than adding 1 day. Use days for calendar logic and hours for elapsed time.