Does a leap day change the count between two dates?
A leap year adds one day: 29 February. The rules:
- A year divisible by 4 is a leap year.
- Century years (ending 00) are leap years only if divisible by 400.
2024 is a leap year (divisible by 4, not a century). 1900 was not a leap year (divisible by 100 but not 400). 2000 was a leap year (divisible by 400).
The 1,047 days between 1 January 2024 and 13 November 2026 includes one leap day (29 February 2024). Without it, the count would be 1,046 days. If your range spans 29 February, add one day to the total.
When people get it wrong: "a year is 365 days" is false in a leap year. A 365-day warranty starting 1 March 2024 expires 29 February 2025, exactly 366 days later. The Duration Calculator tracks this.
How many days between 15 March 2025 and 10 August 2025?
- Days remaining in March: 31 minus 15 equals 16 days (excluding the 15th).
- Full months: April (30), May (31), June (30), July (31). That is four months.
- Days in August: 10 days.
- Total: 16 plus the four full months plus 10.
Why this goes wrong: people forget to subtract the start date from the month's total. "March has 31 days, so 31 minus 15 equals 16" is correct only if you do not count 15 March. If you need to include the start date, add 1 at the end.
Shortcut: use a reference date. 1 January 2025 is day 1 of the year. 15 March is day 74 (31 plus 28 plus 15). 10 August is day 222 (sum the month ends through July, then add 10). Subtract the smaller day-of-year number from the larger one.
Including vs excluding the end date
This is the most common source of disputes.
- Excluding the end date: "30 days from 1 June" ends on 1 July. The count starts on 2 June and ends on 1 July.
- Including the end date: "30 days from 1 June" ends on 30 June. The count starts on 1 June and includes 30 June.
A 7-day free trial starting Monday ends on Sunday (including Monday) or Monday (excluding Monday). The Duration Calculator defaults to excluding the end date, matching most business usage.
How to use the Duration Calculator
Open the Duration Calculator. Enter a start date and an end date. It returns:
- Days (calendar days)
- Weeks and days
- Months and days
- Excluding or including the end date
Enter 1 January 2024 as the start, 13 November 2026 as the end. The tool returns 1,047 days (or 34 months and 12 days). No manual counting needed.
Toggle "include end date" when the wording of your contract, notice period or countdown demands it. Legal documents often say "within 30 days of 1 June." That means the deadline is 30 June (including the end date) or 1 July (excluding it). Check the wording before you toggle.
Work days vs calendar days
Calendar days count every day. Work days exclude weekends and, optionally, public holidays.
- A 14-day notice period starting Thursday: calendar days end Wednesday of week 3. Work days (Monday to Friday) end the following Tuesday.
- A 90-day warranty: calendar days. A 90-work-day warranty: roughly 18 weeks (90 divided by 5).
The Duration Calculator does not calculate work days. For that, use the Work Hours Calculator or a dedicated tool.
Why this matters: employment contracts often specify "calendar days" or "working days." If the contract says "14 days' notice" without qualification, it means calendar days. If it says "14 working days", weekends do not count.
"90 days from signing" is not "three months from signing"
A contract that says "90 days from signing" pins you to a fixed count. "Three months from signing" lands on the same day of the month three months later, which could be 89, 90, 91 or 92 days depending on the months involved. The difference matters for:
- Warranty expiration: a 365-day warranty on a product bought 15 March 2025 expires 15 March 2026, not 14 March. A leap year changes the count.
- Subscription periods: a 30-day free trial that starts 31 January ends 2 March in a common year (28 February plus 2 days), not 2 March in a leap year.
- Notice periods: UK statutory notice of one week means seven calendar days, not five working days.
- Countdowns: "100 days until the wedding" needs an exact date, not a rough month.
The Duration Calculator handles all this automatically. Understanding the manual method helps you spot errors.
How many days should I count for a contract, warranty or countdown?
| Use case | What to count | Example |
|---|---|---|
| Warranty | Calendar days from purchase | 365 days from 15 March 2025 equals 15 March 2026 |
| Subscription | Calendar days from start | 30-day trial from 31 January equals 2 March (common year) |
| Notice period | Calendar days (unless specified) | 7 days from Thursday equals Wednesday |
| Countdown to event | Calendar days excluding start | 100 days before 1 August equals 23 April |
| Project deadline | Work days (often) | 20 work days from 1 June equals 28 June (no holidays) |
Warranty expiration: a warranty that says "1 year from purchase" means 365 days in a common year, 366 in a leap year. "12 months from purchase" means the same day of the month 12 months later, which could be 365 or 366 days depending on the months involved.
Subscription periods: a monthly subscription billed on the 15th renews on the 15th of each month. A 30-day trial does not align with calendar months.
Notice periods: UK employment law requires one week's notice for employees with less than two years' service. That is seven calendar days, not five working days. The Duration Calculator gives the exact end date.
Days in each month
| Month | Days |
|---|---|
| January | 31 |
| February (common) | 28 |
| February (leap) | 29 |
| March | 31 |
| April | 30 |
| May | 31 |
| June | 30 |
| July | 31 |
| August | 31 |
| September | 30 |
| October | 31 |
| November | 30 |
| December | 31 |
Checklist for manual calculation:
- Write the start and end dates.
- Count days remaining in the start month (total days in month minus start day).
- Add full months between.
- Add days in the end month.
- Add a leap day if 29 February falls between the dates.
- Decide: include or exclude the end date. Adjust by 1 if needed.