Add or Subtract Days From a Date
Work out the date a deadline lands on. Add or subtract any combination of days, weeks, months and years, and switch to business-day counting when the contract says working days.
Starting Date
Use negative numbers to move backwards in time.
Month arithmetic is not addition
Adding one month to 31 January cannot land on 31 February. Every system has to pick a rule, and the near-universal choice — the one used here — is to clamp to the last valid day of the target month, giving 28 or 29 February. Adding a further month from that clamped date lands on 31 March in some systems and 28 March in others. This calculator applies months to the original date in one step rather than chaining, which avoids that ambiguity entirely.
When to use business-day mode
Court filings, payment terms, statutory notice periods and service-level agreements are usually written in working days. Counting those on a calendar produces a deadline that is too early, sometimes by several days across a holiday period. Business mode walks forward one working day at a time, skipping Saturdays, Sundays and any holidays you supply, which is exactly how a registry or a finance team will count it.
Order of operations
Years are applied first, then months, then weeks, then days. That order matters near month boundaries: adding "1 month and 30 days" is not the same as adding "30 days and 1 month" when the start date is near the end of a month. Stating the order explicitly means the result is reproducible rather than accidental.
Reading the ISO week number
The ISO week number follows the international standard where week one is the week containing the first Thursday of the year, and weeks always start on Monday. Manufacturing, logistics and European payroll systems schedule by this number, so it is reported alongside the date.