How to Add or Subtract Days From a Date

"What date is 90 days from now?" comes up constantly — a payment due 30 days after invoicing, a return window closing in 28 days, a project milestone 6 weeks out. Counting it on a calendar is slow and easy to get wrong once you cross month boundaries. The arithmetic itself is straightforward; the traps are in the calendar.

Adding and subtracting days

Days are the simplest unit: you move forward or back by that many calendar days, rolling over month and year ends as you go. The only thing to watch is whether you count the start day itself.

That "day zero" convention is where a lot of off-by-one errors creep in. If a 14-day return period starts on the day you receive an item, the last day is the receipt date plus 14 — decide up front whether the start day counts and stay consistent.

Weeks, months and years

• Weeks are the easy case — just days in sevens. Adding 3 weeks is the same as adding 21 days, and the result always lands on the same weekday as the start.
• Months vary in length, so adding a month keeps the same day number where it exists and otherwise rolls into the next month.
• Years mostly keep the same day and month, with one exception: 29 February only exists in leap years.

Why month arithmetic is tricky

Adding "one month" to 31 January is ambiguous — February has no 31st. The standard behaviour, which most calendars and this calculator follow, is to keep the day number and let it spill over, so one month after 31 January lands in early March rather than on an invented 31 February.

This matters for anything billed monthly. If you're relying on exact day counts — interest-free periods, notice periods, deadlines — it's safer to work in days, or to check the rolled-over result rather than assume "the 31st of next month".

Letting a calculator do it

The date add & subtract calculator takes a start date, a direction, an amount and a unit, then shows the resulting date, its day of the week and how many days away it is — handling leap years and month lengths for you. If instead you want the gap between two dates you already know, use the date difference guide, and to skip weekends when counting see counting working days between dates.