Adding and subtracting days:
due dates, deadlines and leap years

What it means to add days to a date

Adding days to a date is simple to describe and surprisingly easy to get wrong in your head. You move forward through the calendar one day at a time, passing through months of different lengths, and you stop after N steps. Because months have 28, 29, 30 or 31 days, the same N produces a different calendar date depending on where you start.

The question matters for real deadlines: a 30-day notice period, a 14-day return window, a 90-day warranty claim, an invoice due in 45 days. Each has a start event and a count, and the answer is a specific date that someone may hold you to.

The first decision is whether the start date counts. In the usual arithmetic convention, the start date is day 0, so 1 January plus 1 day is 2 January. Some contracts and rules count the start day as day 1, which gives an earlier end date, so read how the rule is worded before relying on any tool.

Computers store dates as a running count of days from a fixed origin, which is why date functions rarely get this wrong. Humans, working from memory of the calendar, are the ones who slip, especially near the end of February and across year boundaries.

The rule: start date plus N, with month lengths and leap years

In practice you carry across month boundaries. Add N to the day of the month; if the total passes the month's length, subtract that length, move to the next month, and repeat. Month lengths are 31 days for January, March, May, July, August, October and December, 30 for April, June, September and November, and 28 or 29 for February.

Weeks give a shortcut for the weekday. Because 7 divides evenly, adding 7, 14, 28 or 91 days keeps the same weekday. Adding 90 days is 12 weeks plus 6 days, so the weekday moves forward by six, which is the same as moving back by one.

Worked example: the same 90 days, different answers

Count 90 days from 1 December in three consecutive years. December has 31 days, January has 31, and February has 28 or 29. The result depends on whether February of the following year is a leap February.

A calculator or script that follows the calendar rules does this correctly. A mental shortcut of treating every month as 30 days would put the three answers on different dates and would be wrong for each of them.

Due dates and working backwards

Subtracting days uses the same logic in reverse. To find the start date for a 45-day deadline that falls on a known date, count backwards. Forty-five days before 11 October 2026 is 27 August 2026, a Thursday.

A common use of 280 days is a pregnancy due date estimated from the first day of the last menstrual period. Adding 280 days (40 weeks) to a start of 15 January 2026 gives 22 October 2026, a Thursday. This is a rule-of-thumb estimate, not a diagnosis: a doctor can refine the date from a scan and from the individual's cycle.

The traditional hand method for the same estimate adds 7 days to the start date and then subtracts 3 months. For 15 January that is 22 January and then 22 October of the same year. The two methods agree because 280 days is about 9 months and 7 days.

Keeping the weekday in view

• Add 7 days and the weekday stays the same, so 11 October 2026, a Sunday, plus 14 is another Sunday.
• Add 30 days and the weekday moves forward 2 (30 = 28 + 2).
• Add 365 days and the weekday moves forward 1; add 366 days, as you would across a 29 February to land on the same date, and it moves forward 2.
• Business-day counts exclude weekends and public holidays, so they cannot be done by simple addition.

This last point matters. A calendar-day count and a working-day count give different answers, and holidays differ by state and country. If the deadline is stated in working days, use a tool that handles weekends and holiday lists, or count them by hand from a calendar.

Mistakes that cost a day

Calendar arithmetic goes wrong in a few predictable places. Knowing them is faster than rechecking every answer.

• Counting the start date as day 1 when the rule treats it as day 0.
• Treating one month as 30 days. 31 January 2027 plus 30 days is 2 March 2027, while plus one calendar month is 28 February 2027, the last day of that month.
• Missing a leap day: 365 days from 11 October 2027 is 10 October 2028, not 11 October, because 29 February 2028 lies in between.
• Mixing up day-first and month-first date formats when typing a date, so 03/04 means two different dates.
• Assuming a deadline that ends on a weekend or holiday moves to the next working day. Some rules do this and some do not.

For legal, tax or contractual deadlines, the wording of the document or the statute controls, and a date calculator only gives the arithmetic. If a lot depends on the answer, check with the issuing body or a professional.