Free Date Calculator Online
This free online date calculator tells you the day of week, day of year, ISO week number, and leap-year status for any date. It also lets you add or subtract days to find a new date — completely free, no account needed.
Day of Week
How the Free Date Calculator Works
Pick any date and this free online date calculator immediately returns four pieces of information. First, the day of week — Monday through Sunday — calculated using the Zeller or Tomohiko Sakamoto algorithm under the hood, accurate for any Gregorian calendar date. Second, the day of year, also called the ordinal date: January 1 is day 1, and December 31 is day 365 (or 366 in a leap year).
Third, the ISO 8601 week number. ISO weeks always start on Monday, and Week 1 is defined as the week containing the year's first Thursday. Because of that definition, a few days at the very start or end of a calendar year may belong to a different ISO year. Fourth, whether the year is a leap year — divisible by 4, with century-year exceptions.
Switch to "Add / Subtract Days" mode to do date arithmetic. Enter a start date and a number of days, choose whether to add or subtract, and the free date calculator finds the resulting date — handling month-end rollovers correctly. For example: 90 days after March 1, 2025 is May 30, 2025. This online tool handles all the edge cases automatically.
When a Free Online Date Calculator Is Useful
Deadlines
Add 30 or 60 days to a contract start date to find the exact deadline day — including what day of the week it falls on. Calculate online in seconds for free.
Historical Research
Determine what day of the week any past event occurred. Neil Armstrong stepped onto the moon on Sunday, July 20, 1969. Look up any date free online instantly.
Payroll & Invoicing
Net-30 or net-60 payment terms are exact-day calculations. This free date calculator makes those instant and error-free.
Days in Each Month
| Month | Days | Notes |
|---|---|---|
| January | 31 | Always 31 days |
| February | 28 / 29 | 29 days in leap years |
| March | 31 | Always 31 days |
| April | 30 | Always 30 days |
| May | 31 | Always 31 days |
| June | 30 | Always 30 days |
| July | 31 | Always 31 days |
| August | 31 | Always 31 days |
| September | 30 | Always 30 days |
| October | 31 | Always 31 days |
| November | 30 | Always 30 days |
| December | 31 | Always 31 days |
Leap Year Rules Explained
The Gregorian calendar adds an extra day (February 29) to keep the civil year synchronized with the solar year of ~365.2422 days. The rules: a year is a leap year if it is divisible by 4. Exception: century years (divisible by 100) are not leap years. Exception to the exception: years divisible by 400 are still leap years.
So 2000 was a leap year (divisible by 400), 1900 was not (divisible by 100 but not 400), and 2024 is a leap year (divisible by 4, not a century year). The next century non-leap year is 2100. Enter any year into this free online date calculator above to verify instantly.
ISO Week Numbers — Key Facts
ISO 8601 week numbers differ from simple calendar weeks. Week 1 is the week containing the first Thursday of the year. This means January 1 can belong to the last ISO week of the prior year if it falls on a Friday, Saturday, or Sunday. Similarly, December 31 can fall in ISO Week 1 of the following year.
Most years have 52 ISO weeks; a year has 53 weeks if January 1 or December 31 is a Thursday, or if it is a leap year and either of those days is a Wednesday. The date calculator shows the correct ISO week for any date.
Date Calculator Tips
365
Days in a regular year. Add 1 for a leap year. The date calculator handles this automatically.
52–53
ISO weeks per year. Most years have 52 weeks; about 71 out of every 400 years have 53.
97/400
Leap years in every 400-year Gregorian cycle. That keeps the calendar accurate to within 1 day per 3,030 years.
Mon
ISO weeks always start on Monday. If you need Sunday-based weeks, the ordinal day calculation changes by one.
Frequently Asked Questions
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