How to count the days between two dates

How many days since the wedding, how long the vacation lasts, how many days are left before the deadline — it is all the same task: count the days between two dates. People usually slip in two places: the length of the months, and whether to count the first and last day. Here is how to get both right, with a worked example.

The key rule: inclusive or not

The difference between two dates is how many times you turn the calendar page to get from one to the other. From March 1 to March 2 is 1 day; from March 1 to March 10 is 9 days. That is the answer to “how many days has it been since” and “how many days until.”

But when you count how long something lasts and both end days are part of it — a vacation, a business trip, sick leave — add one. A vacation from July 1 through July 14 is 14 days, even though the dates are 13 days apart.

How many days are in each month

The second common mistake is treating every month as 30 days. Here is the exact table:

The knuckle trick helps: make a fist and count the months across your knuckles and the dips between them. January is a knuckle (31), February a dip (short), March a knuckle (31) and so on; July and August land on two knuckles in a row, so both have 31 days. Or use the rhyme “Thirty days hath September, April, June and November.”

Example: days from May 15, 2025 to October 27, 2026

Work in big steps: whole years first, then months, then days.

1. One whole year: May 15, 2025 to May 15, 2026 — 365 days (no February 29 in between).
2. The rest of May 2026: May 15 to May 31 — 16 days.
3. Whole months: June 30 + July 31 + August 31 + September 30 = 122 days.
4. October: September 30 to October 27 — 27 days.
5. Total: 365 + 16 + 122 + 27 = 530 days.

If both end dates should count, it is 531 days.

How to do it in Excel or Google Sheets

Spreadsheets store dates as numbers, so the difference is plain subtraction. With the start date in A1 and the end date in B1:

• =B1-A1 — days between the dates (format the cell as a number, not a date);
• =B1-A1+1 — the same, inclusive;
• =DATEDIF(A1,B1,"d") — days; "m" — whole months; "y" — whole years;
• =TODAY()-A1 — how many days since a date, as of today;
• =NETWORKDAYS(A1,B1) — working days, Monday to Friday (pass a range of holidays as the third argument).

Common mistakes

• Forgetting February 29. If the range includes February 29 of a leap year (the next one is 2028), add a day. How to spot a leap year is in how many days are in a year.
• Mixing up “since” and “lasts.” For deadlines and contracts where every calendar day matters, check the rule — official documents often count inclusively.
• Using 30-day months. Fine for a rough figure, not for an exact one: over a year the error adds up to five days.

So you don’t have to recount every time

Counting the days once is easy. It gets tedious when you want the number every day: how long you have been together, how long since your last cigarette, how long until the trip. A counter that updates itself is simpler.

In Temfy: Countdown Days & Widget you set a date and choose a direction — until or since. The app shows days, hours, minutes and seconds, and a Home Screen widget keeps the number in sight. Count-ups get milestones — 100, 500 and 1,000 days and anniversaries. The app is free for iPhone and Android, no sign-up needed.