The One-Line Method
The number of days between two dates is simply the later date minus the earlier date. A calculator, or a spreadsheet that stores dates as day numbers, handles month lengths and leap years for you. The hard part is not the subtraction; it is deciding which days count.
- Exclusive (a difference): count the start date but not the end date. Monday to Friday is 4 days. This is what "days between" usually means.
- Inclusive: count both the start and the end date. Monday to Friday is 5 days. Use it when every calendar day touched counts, such as the days of a trip or a rental billed per calendar day.
The two answers always differ by exactly one day, which is why so many off-by-one mistakes happen on deadlines, leases and countdowns.
Month Lengths and the Leap-Year Rule
Counting by hand goes wrong at month boundaries. Seven months have 31 days, four have 30, and February has 28 or 29. The Gregorian calendar's leap-year rule, as published by the U.S. Naval Observatory and NIST, is:
| Year is… | Leap year? |
|---|---|
| Divisible by 4 (e.g. 2028) | Yes |
| Divisible by 100 (e.g. 1900, 2100) | No |
| Divisible by 400 (e.g. 2000) | Yes |
NIST explains why: the actual year is about 365.242 days, so adding a day every four years over-corrects slightly, and skipping three century leap years every 400 years brings the calendar back in line. The practical effect is that a one-year span is 365 days, or 366 if it contains a February 29: January 1, 2025 to January 1, 2026 is 365 days, while February 1, 2024 to March 1, 2025 is 394 days because it crosses February 29, 2024.
Calendar Days vs. Business Days
Many real deadlines are in business days. A quick estimate is that five of every seven days are weekdays, but the exact count depends on where the weekends fall, so count them rather than multiplying. Public holidays are a second layer: a weekday count does not know about them, so subtract any holidays that fall in the range yourself.
Worked Example: Countdown from Oct 21 to Dec 25, 2026
Start: Wednesday, October 21, 2026. End: Friday, December 25, 2026.
- October 21 to October 31: 11 days (counting Oct 21, not counting the end)
- November: 30 days
- December 1 to 24: 24 days
Total: 65 days, or 66 inclusive. That is 9 weeks and 2 days. Of those 65 days, 47 are weekdays (Monday to Friday), before you subtract holidays such as Thanksgiving.
How Courts Count a Deadline
Legal deadlines show why the counting rule matters. Rule 6(a) of the Federal Rules of Civil Procedure says that for a period stated in days you exclude the day of the event that triggers the period, count every day including weekends and holidays in between, and include the last day. If the last day is a Saturday, Sunday or legal holiday, the period runs to the next day that is not.
So a 30-day period triggered on Wednesday, October 21, 2026 starts counting on October 22 and ends on Friday, November 20, 2026, which is a business day, so that is the deadline. State courts, contracts and agencies can use different rules, so always check the rule that applies to you; this is an illustration, not legal advice.
Tips for Getting It Right
- Decide inclusive or exclusive first and write it down next to the answer.
- Use dates, not "about a month". Months run 28 to 31 days, so "one month" and "30 days" give different end dates.
- Watch February 29 in any span that crosses a leap year.
- For business days, list the holidays that apply to you; they vary by country, state and employer.
Frequently Asked Questions
How do I count the days between two dates?
Subtract the earlier date from the later one. That gives the exclusive count, which includes the start date but not the end date. Add one if both dates should count.
Does the days between dates count include the end date?
Not by default. "Days between" is usually exclusive, so Monday to Friday is 4 days. Inclusive counting, which counts both dates, gives 5.
Which years are leap years?
Years divisible by 4 are leap years, except century years, which are leap years only if divisible by 400. So 2000 was a leap year, 1900 was not and 2100 will not be.
How many days are between October 21 and December 25, 2026?
65 days, or 66 if you count both dates. 47 of those days are weekdays, before subtracting holidays.
How do federal courts count a 30-day deadline?
Under Federal Rule of Civil Procedure 6(a), exclude the day of the triggering event, count every day after it including weekends, and include the last day; if the last day is a weekend or legal holiday, the deadline moves to the next business day.
Sources
- U.S. Naval Observatory, "Introduction to Calendars" (aa.usno.navy.mil/faq/calendars), Gregorian leap-year rule.
- NIST Time and Frequency Division, "Time and Frequency from A to Z: Leap Year" (nist.gov), length of the year and century-year rule.
- Federal Rules of Civil Procedure, Rule 6(a), Computing Time (law.cornell.edu/rules/frcp/rule_6).
This guide is for education. For legal or contractual deadlines, check the rule that governs your situation.