Date differences
without the guesswork

Inclusive or exclusive: do you count the first day?

Counting days looks trivial until a form asks for the exact number. Three things cause most wrong answers: whether the first day counts, how long each month is, and whether weekends are included.

Take the first one. The plain difference, called exclusive counting, leaves the start day out: from 10 June to 11 June is 1 day. Inclusive counting includes both ends, so the same dates give 2 days.

Most calculators, ours included, give the plain difference: end date minus start date. Add 1 when a form, contract or leave application says both days are included. When the wording is unclear, ask or check the document.

For example, take leave from 5 March to 7 March 2026. The plain difference is 2 days, but if the first and last day both count as leave, you are away for 3. When late fees or interest are charged per day, that one day can change the amount.

Months are uneven, and leap years add a day

A year is a leap year if it is divisible by 4, except for century years, which must also be divisible by 400. So 2024 and 2028 are leap years, 2000 was one, but 1900 and 2100 are not. A normal year has 365 days and a leap year has 366.

This is why 28 February to 1 March is 1 day in 2026 but 2 days in 2024. Over a full year it adds up: 1 January 2026 to 1 January 2027 is 365 days, but 1 January 2024 to 1 January 2025 is 366.

A quick memory aid: make a fist and count the months along your knuckles and the gaps between them. Each knuckle is a 31-day month and each gap is a shorter one. Start again on the first knuckle for August.

Turning days into weeks and months

Months are harder because a month is not a fixed number of days. Two months after 15 January is 15 March, 59 days later in 2026. Two months after 15 March is 15 May, 61 days later. For a rough conversion, a year is about 52 weeks and a day, and an average month is about 30.4 days (365 ÷ 12).

Month ends are a trap. There is no 31 February, so one month after 31 January has no single obvious answer, and tools and contracts handle it differently. For a precise deadline, count exact days instead of months.

Worked example: 15 January to 20 March 2026

Count the days left in the start month, add the full months in between, then add the days elapsed in the end month.

Check it a second way: 15 January to 15 March is 31 + 28 = 59 days, and 15 March to 20 March adds 5 more, which is also 64.

Adding days, and a caution on business days

Counting forward works the same way, which helps with invoice due dates, return windows, notice periods and trial end dates. An invoice dated 1 March 2026 with payment due in 30 days falls due on 31 March 2026. 45 days after 15 January 2026 is 1 March 2026.

Notice periods show the inclusive question again. A 30-day notice given on 10 November 2026 ends on 10 December 2026 if the first day is not counted, and a day earlier if it is. The Add Days to a Date Calculator does the counting for you.

The Age Calculator applies the same logic to a date of birth, but counts only completed years. Someone born on 15 January 2000 is 25 on 14 January 2026 and turns 26 the next day.