Add or subtract days from a date

Date arithmetic

Adding days and weeks is simple: 90 days after a date is exactly 90 calendar days later. Adding months and years is trickier because months have different lengths. The usual rule, used by spreadsheets and most programming languages, keeps the same day of the month and, if that day doesn't exist, uses the last day of the month.

Worked examples

• January 31 + 1 month = February 28, 2026 (a Saturday), the 59th day of the year.
• February 29, 2028 + 1 year = February 28, 2029.
• 10 business days after Friday, October 16, 2026 = Friday, October 30, skipping two weekends.
• 90 days before October 17, 2026 = July 19, 2026.

Common uses

• Return windows and warranties ("30 days from purchase").
• Notice periods and contract renewals.
• Medication schedules and follow-up appointments.
• Project deadlines in business days.

For deadlines set by law or contract, check whether the first day counts and whether a deadline falling on a weekend or holiday moves to the next business day.

Days and weeks are added as whole calendar days. For months and years, the page moves the month and year and keeps the day of the month, using the last day of the month when the original day does not exist. Business days are added one at a time, skipping Saturdays and Sundays. The weekday and day of the year are calculated for the result. Holidays are not skipped.