Add/Subtract Days
Add or subtract a number of days from a date to get the resulting date, its weekday and its ISO week number.
Quick Presets
Updated Formula covered by automated tests
Enter a starting date and a number of days, choose whether to go forward or back, and the tool returns the landing date along with its weekday and ISO week number. It answers questions like "what is 90 days from March 8?", "when does a 30-day return window close?" or "what date was 180 days before the hearing?"
Deadlines expressed in days show up everywhere: medication courses, probation periods, notice to vacate, the 60-day window to roll over a retirement distribution, visa stays counted in days, or a lab sample that must be processed within 14 days. Counting on a wall calendar is slow and month lengths make mistakes easy, especially when the span crosses February or a year boundary.
The formula
- N(start)
- Serial day number of the starting date (days since a fixed epoch)
- d
- Number of calendar days to add (+) or subtract (−); weekends and holidays are counted like any other day
- date(n)
- Converts a serial day number back to year, month and day
- Thursday of that week
- The Thursday in the same Monday-to-Sunday week as the result; ISO 8601 assigns a week to the year that contains its Thursday
Adding days by hand means walking through month lengths: 31 for January, 28 or 29 for February, and so on. Converting the date to a serial day number turns the problem into ordinary addition, and converting back handles every month length and leap day automatically. Day 0 is the start date itself, so adding 1 to a Saturday gives Sunday.
The week number follows ISO 8601, used across Europe and in most business software: weeks start on Monday and week 1 is the one containing the first Thursday of January. That is why January 1, 2027, a Friday, belongs to week 53 of 2026. US wall calendars that start weeks on Sunday can show a different number.
Worked example
A pharmacy dispenses a 90-day supply on Sunday, March 8, 2026, the same day US clocks spring forward. The pharmacist wants the date the supply runs out, so the refill can be scheduled.
- 1 Days remaining in March after the 8th: 31 − 8 = 23, which brings us to March 31.
- 2 Add April (30 days): 23 + 30 = 53, reaching April 30.
- 3 Add May (31 days): 53 + 31 = 84, reaching May 31.
- 4 The remaining 90 − 84 = 6 days land on June 6, 2026.
- 5 June 6, 2026 is a Saturday in ISO week 23; the 23-hour day on March 8 does not change the date count.
Result: Ninety days after March 8, 2026 is Saturday, June 6, 2026 (ISO week 23). Going back 90 days instead gives Monday, December 8, 2025.
Before you rely on the result
- Results use the Gregorian calendar and whole days; time of day and time zones are not considered.
- Legal, medical and financial deadlines often have their own counting rules (excluded days, business-day roll-forward). Confirm the rule with the issuing authority before relying on a date.
Frequently asked questions
Is "within 30 days" counted from the day of the event or the day after?
It depends on who sets the rule. US federal court rules (FRCP 6) exclude the day of the triggering event and count every day after, including weekends, and extend a deadline that lands on a weekend or legal holiday to the next working day. A store return policy may count the purchase day as day 1. This tool treats the start date as day 0, which matches the court-style count.
How do I count only weekdays instead of calendar days?
This page counts every calendar day. For a deadline measured in business days, use the project timeline or working days calculators on this site, which skip Saturdays, Sundays and any holidays you list.
Why is my week number one higher or lower than my calendar shows?
Calendars printed in the US often number weeks starting on Sunday with week 1 containing January 1. The ISO system used here starts weeks on Monday and assigns the week containing the first Thursday as week 1. Around New Year the two systems can disagree by one week.
What happens when the count passes February 29?
Leap days are counted like any other day. Adding 365 days to January 1, 2028 lands on December 31, 2028, not January 1, 2029, because 2028 has 366 days.