Date Calculator
Calculate the difference between two dates in days, weeks, and months.
About this calculator
This calculator finds the difference between two calendar dates. Enter any two dates to see how many days, weeks, and approximate months separate them. Useful for project planning, age calculation, deadlines, and more.
Common Date Calculations
Quick reference for frequently needed calendar facts.
| Unit | Value | Notes |
|---|---|---|
| Days in a week | 7 | Fixed across all calendar systems |
| Days in a month | 28 – 31 | February has 28 days (29 in a leap year); all others 30 or 31 |
| Days in a year | 365 or 366 | 366 in a leap year, which occurs every 4 years (with exceptions) |
| Weeks in a year | 52.18 | 365 / 7 = 52.14; most years contain 52 full weeks plus 1 or 2 extra days |
| Days in a decade | 3,652 or 3,653 | Depends on how many leap years fall within the 10-year span |
| Days in a century | 36,524 or 36,525 | Century years (e.g. 1900) are not leap years unless divisible by 400 (e.g. 2000) |
Real-World Date Calculation Examples
Enter the start and end dates above to calculate any of these common scenarios.
Enter today as the start date and your planned retirement date as the end date. The result shows exactly how many days you have left - Motivating for some, sobering for others. You can also use the Countdown to see it tick down in real time.
How many days ago did World War II end? Enter September 2, 1945 as the start date and today's date as the end date. The calculator returns the exact number of days, weeks, and approximate months since that moment. Explore more historical events with exact day counts.
Calculate how many days elapsed between a product launch and its first major update, or between signing a contract and its expiry date. Useful for reporting KPIs and measuring project cycles.
Enter the first day of the last menstrual period as the start date and today (or the estimated due date) as the end date. Divide the number of days by 7 to get gestational age in weeks. Most pregnancies are approximately 280 days (40 weeks) from that reference point.
If a lease runs from March 1 to February 28 next year, the exact number of days matters for prorating rent or calculating notice periods. The calculator gives you the precise answer, accounting for the specific calendar months involved.
Calendar Systems Explained
The Gregorian calendar, which this calculator uses, was introduced by Pope Gregory XIII in October 1582 as a reform of the Julian calendar. The Julian calendar had a slight error in its leap year rule that caused it to drift relative to the solar year by about 11 minutes per year - Accumulating to roughly 10 days of drift by the 16th century. Gregory's reform corrected this by adding the rule that century years are only leap years if divisible by 400. The Gregorian calendar is now used by most of the world as the standard civil calendar, although other systems (Islamic, Hebrew, Chinese, Julian) remain in use for religious and cultural purposes. For how different countries handle clock changes within this calendar, see the time zones index.
February's short length is a legacy of the Roman calendar, which originally had only 10 months and began in March. When January and February were added to the beginning of the year, their lengths were set to fill out the remaining days. Political interventions - Particularly Julius Caesar and Augustus Caesar each adjusting months named after themselves - Resulted in the irregular month lengths we still live with. Leap years exist because the Earth does not orbit the sun in a round number of days. The actual solar year is approximately 365.2422 days, so adding one full day every four years (with the century correction) keeps the calendar closely aligned with the astronomical seasons over time.
Days and Dates Trivia
- 01 The Gregorian calendar drifts from the true solar year by approximately 1 day every 3,236 years. This means a further calendar reform will eventually be needed, though not for a very long time.
- 02 September is the longest month name in English but derives from the Latin septem ("seven") - It was the seventh month in the original Roman calendar before January and February were prepended to the year.
- 03 Friday the 13th occurs between one and three times in any given calendar year. The 13th falls on a Friday more often than on any other day of the week over a 400-year Gregorian cycle.
- 04 The Doomsday algorithm, developed by mathematician John Horton Conway, lets you calculate the day of the week for any date in your head. It relies on the fact that certain "doomsday" dates (4/4, 6/6, 8/8, 10/10, 12/12, and others) always fall on the same day of the week within a given year.
- 05 The seven-day week has no astronomical basis - Unlike months (lunar cycle) and years (solar cycle). The seven-day cycle was popularised by the Romans and the Judeo-Christian tradition, and is now universal despite being entirely a human convention.
Frequently Asked Questions
How do I calculate business days?
This calculator returns calendar days - The raw count of days between two dates, including weekends and public holidays. To calculate business days, subtract the number of Saturdays and Sundays in the period (roughly 2 per 7 days, or about 28.6% of the total). For a more precise count that excludes specific public holidays, you would need a country-specific business day calculator.
Does the calculator include the end date?
The calculator counts the number of days between the two dates, not including the end date itself. For example, January 1 to January 3 returns 2 days - Reflecting the two intervals (Jan 1 to Jan 2, and Jan 2 to Jan 3). If you want to include both endpoints, add 1 to the result.
How are leap years accounted for?
The calculator uses the Gregorian leap year rules: a year is a leap year if it is divisible by 4, except for century years, which must also be divisible by 400. So 2000 was a leap year, but 1900 was not. When your date range spans February 29 of a leap year, that day is counted as a normal calendar day, adding one to the total count compared to a non-leap year interval of the same calendar span.
What is the difference between calendar days and business days?
Calendar days count every day from the start date to the end date, including weekends and public holidays. Business days (also called working days) count only Monday through Friday, excluding public holidays. A 14-calendar-day deadline includes two weekends; the same period has only 10 business days. Contracts, shipping estimates, and legal deadlines often specify which they mean - Always clarify which applies to your situation.