How adding to a date works
Days and weeks are simple: the calculator steps that many calendar days forward or backward, weekends included, crossing month and leap-year boundaries naturally. Months and years walk the calendar instead of adding a fixed number of days: "one month later" means the same day-of-month in the next month, and "one year later" means the same date next year.
End-of-month clamping
When the target month is shorter than your starting day, the result is clamped to the last day of that month. 31 January + 1 month = 28 February (29 February in a leap year), because February has no 31st. Likewise 31 March + 1 month = 30 April, and 29 February 2024 + 1 year = 28 February 2025. This matches how spreadsheets and most billing systems behave, so a monthly subscription started on the 31st renews on the last day of shorter months.
Worked example
Base date 19 July 2026, add 90 days: stepping through the rest of July (12 days), August (31), September (30) and 17 days of October lands on Saturday, 17 October 2026 — exactly 90 calendar days later. Adding 3 months instead gives 19 October 2026, which is 92 days, because August and July are 31-day months.
Common uses
- Deadlines: what date is 30 / 60 / 90 days from an invoice or contract date.
- Reminders: 2 weeks before an event, 6 months after a purchase (warranty checks).
- Planning backwards: subtract 9 months, or count back 45 days from a due date.