The short answer
- The start date is day 0, so 1 day after Monday is Tuesday.
- Days and weeks are exact: a week is always 7 days.
- Months follow the calendar, and a date that does not exist (such as 31 February) becomes the month’s last day.
- Working days skip Saturdays, Sundays and, in the UK, the bank holidays of the nation you choose.
Does the start date count?
The calculator treats the start date as day 0. Adding 1 day to Monday gives Tuesday, and adding 7 days gives the next Monday. This matches how most deadlines are written: “within 14 days of 1 March” usually means by 15 March.
Check the wording
Some rules count the first day, or say “clear days”, which leave out both the first and the last day. If a deadline is important, read the exact words and, if in doubt, aim for the earlier date.
Adding days and weeks
Days and weeks are the simplest sums, because every day and every week is the same length. A few examples from 11 October 2026:
| Add | Date | Day |
|---|---|---|
| 6 weeks | 22 November 2026 | Sunday |
| 90 days | 9 January 2027 | Saturday |
| 13 weeks | 10 January 2027 | Sunday |
| 1,000 days | 7 July 2029 | Saturday |
Adding whole weeks always lands on the same weekday as the start date, which is handy for repeating meetings or shifts.
Adding months
A month is not a fixed number of days, so months are added by the calendar: the same day number, a set number of months later. 3 months after 11 October 2026 is 11 January 2027. That is 92 days, while 3 months from another start could be 89 to 92 days.
- From
- 11 October 2026
- To
- 11 January 2027
- Rule
- Same day, 3 months on
- From
- 11 October 2026
- To
- 10 January 2027
- Rule
- 91 days exactly
Month ends and 31st dates
If the start date is the 29th, 30th or 31st, the target month may be too short. The calculator then uses the last day of that month, which is the usual rule in contracts and bank payments.
- Count 6 monthsSeptember to FebruaryFebruary 2027
- 31 February does not existFebruary 2027 has 28 daysUse 28
Likewise, 1 month after 31 January 2026 is 28 February 2026, but 1 month after 31 January 2024 was 29 February 2024, a leap year.
Adding years and leap years
Adding a year keeps the same day and month. The one exception is 29 February: a year after 29 February 2024 is 28 February 2025. Leap years come every 4 years, except century years that do not divide by 400, so 2000 was a leap year and 2100 will not be.
Mixing years, months and days
When you enter several units, the calculator adds the years and months first, then the weeks and days. Doing it in this order gives the answer people expect, and the step-by-step card shows each stage.
- Take away 14 months11 August 2025
- Take away 10 days1 week and 3 days1 August 2025
Counting back from a date
Subtracting works the same way in reverse. It answers questions like “when must I post this?”, “when did the 90-day period start?” or “when should I hand in my notice?”. 100 days before 11 October 2026 is Friday 3 July 2026.
Working days
A working day (or business day) is normally Monday to Friday, not counting public holidays. Choose Working days in the calculator to count only these. The start date is day 0, and the next working day is day 1, so 1 working day after a Friday is the Monday.
- Working days counted20
- Weekend days skipped6
- Bank holidays skipped0
That is 26 calendar days later. Over a typical month, working days are roughly 5 in every 7 calendar days.
UK bank holidays by nation
England and Wales, Scotland and Northern Ireland each have their own bank holidays. When a bank holiday falls on a weekend, a substitute day is usually given on the next weekday. The 2027 dates for England and Wales are:
| Holiday | Date |
|---|---|
| New Year's Day | Friday 1 January |
| Good Friday | Friday 26 March |
| Easter Monday | Monday 29 March |
| Early May bank holiday | Monday 3 May |
| Spring bank holiday | Monday 31 May |
| Summer bank holiday | Monday 30 August |
| Christmas Day (substitute day) | Monday 27 December |
| Boxing Day (substitute day) | Tuesday 28 December |
Scotland adds 2 January (a substitute day on 4 January in 2027) and St Andrew’s Day, has its summer holiday on the first Monday of August, and does not have Easter Monday. Northern Ireland adds St Patrick’s Day (17 March) and the Battle of the Boyne (12 July). The bank holiday calculator lists every date for each nation.
Working days around Christmas and Easter
Holiday periods make the biggest difference to working-day sums. 10 working days after Thursday 17 December 2026 lands on:
| Rule | Result | Holidays skipped |
|---|---|---|
| Weekends only | Thursday 31 December 2026 | 0 |
| England and Wales | Tuesday 5 January 2027 | 3 |
| Scotland | Wednesday 6 January 2027 | 4 |
At Easter, 5 working days after Wednesday 24 March 2027 is Friday 2 April 2027 in England and Wales, because Good Friday and Easter Monday are skipped. In Northern Ireland, 5 working days after Friday 12 March 2027 is Monday 22 March, one day later than elsewhere because of St Patrick’s Day.
Working days outside the UK
The calculator knows UK bank holidays only. For other countries, choose “None (weekends only)” under More options, then add one working day for each public holiday in your country that falls within the period. In countries where the weekend is not Saturday and Sunday, count by hand or use calendar days.
Deadlines and notice periods
Common uses for a date calculator include:
- Notice periods. A one-month notice period handed in on 15 March ends on 15 April. See the notice period and final pay calculator for pay.
- Refunds and returns. In the UK, you usually have 14 days to cancel most online orders after the goods arrive.
- Delivery and payment terms. “Payment within 30 days” and “delivery in 5 working days” are both easy to check.
- Travel. Some countries require a passport to be valid for 3 or 6 months after you arrive.
Legal deadlines
Courts, tribunals and government bodies have their own counting rules. Use the calculator as a guide and check the rules for your case.
Repeating dates and schedules
The “Repeating the period” table shows the next 8 dates at the same interval, such as every 28 days for a prescription or every 3 months for a bill. Each date is worked out from the start date, not from the one before, so a monthly schedule from 31 January goes 28 February, 31 March, 30 April rather than drifting to the 28th.
Week numbers and day of the year
The calculator shows the ISO week number and the day of the year for the result. ISO weeks start on Monday, and week 1 is the week that contains the year’s first Thursday. Some years have 53 weeks: 2026 is one, so 31 December 2026 and 1 January 2027 are both in week 53. 11 October 2026 is in week 41 and is day 284 of the year.
Writing dates clearly
In the UK, 06/10/2026 means 6 October 2026, but in the US it means June 10. To avoid mistakes, write the month as a word (6 October 2026) or use the international format, 2026-10-06, which also sorts correctly in spreadsheets and file names.
Date sums in a spreadsheet
In Excel or Google Sheets, =A1+90 adds 90 days to a date in A1, =EDATE(A1, 3) adds 3 months with the month-end rule, and =WORKDAY(A1, 10, holidays) adds 10 working days, skipping dates listed in a holidays range. =NETWORKDAYS(A1, B1, holidays) counts working days between two dates, including both ends.
Common mistakes
- Counting the start date as day 1, which makes every answer a day early.
- Treating a month as 30 days, which drifts by a day or two each month.
- Forgetting a bank holiday, especially substitute days after Christmas and New Year.
- Using England’s bank holidays for Scotland or Northern Ireland.
- Mixing up UK and US date order.
Probation, trial and contract periods
Many work and contract periods are written in months or weeks, and the start date decides where they end. A few examples, each counted with the start date as day 0:
| Period | Starts | Ends |
|---|---|---|
| 6-month probation | 1 September 2026 | 1 March 2027 |
| 3 months' notice | 31 October 2026 | 31 January 2027 |
| 12-week contract | 1 October 2026 | 24 December 2026 |
Some contracts end on the day before the matching date (for example, a year from 1 September ending on 31 August). If yours is written that way, take one day off the result. Your contract or offer letter says which applies.