Add or subtract days, weeks, months or years from any date — or count business days and skip weekends and US federal holidays. The table underneath shows the 30, 45, 60, 90, 120 and 180 day marks straight away.
Pick a start date and an amount.
These are the offsets that come up most often — notice periods, return windows, warranty terms, payment deadlines and probation periods. Change the start date above and the whole table recalculates.
| Offset | Calendar days | Business days |
|---|
The two right-hand columns often diverge by weeks. Thirty calendar days is a month; thirty business days is six weeks. Whenever a document says "30 days", check which of the two it means before you agree to it.
The method that produces the fewest errors is to move one month at a time rather than adding everything at once.
This is exactly what the calculator does, with one addition it never forgets: leap years. February 2028 has 29 days, and a span crossing it is one day longer than the same span crossing February 2027.
Adding 30 days moves the day count by 30. Adding one month advances the calendar month and then clamps if the target month is shorter. The two give different answers, and the gap compounds across recurring dates.
| Start | + 30 days | + 1 month |
|---|---|---|
| 31 January 2026 | 2 March 2026 | 28 February 2026 |
| 31 March 2026 | 30 April 2026 | 30 April 2026 |
| 15 October 2026 | 14 November 2026 | 15 November 2026 |
| 30 November 2026 | 30 December 2026 | 30 December 2026 |
When the target month is shorter than the starting day number, the result is clamped to that month's last day. 31 January plus one month is 28 February in a common year and 29 February in a leap year. 31 March plus one month is 30 April, because April has only thirty days.
This matters for anything that recurs. A subscription that renews on the 31st cannot renew on 31 February, so it lands on the 28th, and the renewal date then sticks there for the rest of the year. The same thing happens to invoicing cycles, lease anniversaries and warranty expiries.
Calendar days count every day. Business days skip Saturdays and Sundays, and in most definitions public holidays as well. The difference is large enough to change a plan: thirty calendar days from a Friday ends on a Sunday, while thirty business days ends six weeks later and on a weekday.
Subtracting works the same way in reverse, and subtracting business days correctly walks backwards past weekends rather than simply snapping to the nearest weekday. Going back 30 business days from a Monday does not land on the same weekday as going forward 30, because the weekends crossed are different.
Adding 30 days to 1 October gives 31 October. The start day is not counted and the end day is. Some documents count both endpoints as elapsed days, which adds one to every result. It is a small convention with a large consequence: a one-day difference can decide whether a filing is late.
A purchase made on 19 September 2026 with a 90 day return window closes on 18 December 2026. Count it as three months instead and the date becomes 19 December — one day later, and enough to void a return made on the 19th.
If the policy says 90 business days rather than 90 days, the window runs to roughly 25 January 2027, more than a month longer. Read the wording before assuming which one applies.
A tenancy requires 45 days written notice, and the tenant serves notice on Tuesday 20 October 2026. Forty-five calendar days later is 4 December 2026, a Friday. Count 45 business days instead and the period ends around 22 December — eighteen days later, and past the point some agreements allow notice to be served for that month.
A grant application closes on Friday 15 January 2027 and the funding body requires the supporting documents to be submitted at least 30 business days beforehand. Switch the direction to subtract, tick business days, and 30 business days before 15 January 2027 is 4 December 2026 — which crosses both Christmas Day and New Year's Day, so ticking the holiday exclusion moves it to 8 December.
Planning backwards from a fixed deadline is where the business-day count earns its keep. Doing it in calendar days would have suggested 16 December and left the application three weeks short.
Tying a deadline to the federal calendar? The 2027 federal holiday calendar lists every observed date, including the two that move.
Date arithmetic follows the Gregorian calendar and ISO 8601 date notation. All calculations run on calendar dates with no clock time attached, so results never shift because of a time zone or daylight saving change.
Holiday table and reference dates last verified: 19 September 2026.
This tool provides calculations, not legal advice. Statutory deadlines can follow their own counting rules and holiday calendars.
Set the start date to today and read the 90 day row of the table above — it updates as soon as the date changes. Ninety days is roughly three months, but because calendar months vary between 28 and 31 days, counting in days and counting in months give different answers.
Add the number of days to the day number, subtract the month length when the total exceeds it, and step forward a month. Repeat until the day fits. Check February specifically, and work one month at a time rather than adding everything at once.
No. From 31 January, 30 days later is 2 March while one month later is 28 February in a common year and 29 February in a leap year.
The result is clamped to that month's last day. 31 January plus one month becomes 28 or 29 February, and 31 March plus one month becomes 30 April. This convention applies to leases, contracts and recurring billing.
Tick the business days box. Saturdays and Sundays are skipped, and the US federal holidays too if you enable that option. Thirty calendar days from a Friday lands on a Sunday; thirty business days lands six weeks later on a weekday.
No. Adding 30 days to 1 October gives 31 October, a 30 day span with the start day excluded and the end day included. If a document counts both endpoints, add one day.
Approximately three. The exact answer depends on which months the span crosses, because three consecutive months can hold 89, 90, 91 or 92 days. If a contract says 90 days, count days.
Almost always the start-day convention or a leap year. Decide whether the start date counts, write it down, and check February. The calculator shows how many calendar days it moved so you can see the convention applied.
Set the direction to subtract. Subtracting business days correctly walks backwards past weekends rather than landing on the nearest weekday, which is what you need when planning from a fixed deadline.
It is a useful cross-check, but statutory deadlines often follow their own rules — excluding the trigger day, rolling to the next business day, or using a jurisdiction-specific holiday calendar. Confirm against the governing document.
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