✓ 22 Free Date Tools ✓ No Registration ✓ Leap Year Aware ✓ Works on All Devices
Date math is one of the most common everyday calculations — yet most online tools give you a number with no explanation, no timezone awareness, and no business-day conventions. Our date calculators are different. Every tool uses the Gregorian calendar (the international standard) and runs calculations in UTC per ISO 8601. For work-day math, we follow the ISO 8601 week numbering standard. For age calculations, we use exact day-counting — not the rough "year = 365 days" approximation.
📅 Date Math Tools
Add, subtract, and count days, weeks, months, and years.
📅 Calendar & Lookup Tools
Find days, weeks, and date facts.
👦 Age & People
Age and life-event calculators.
⏰ Time Unit Conversions
Convert between time units and clocks.
About Date Calculations
Our date calculators use the Gregorian calendar, the international standard for civil use. The Gregorian calendar was introduced by Pope Gregory XIII in October 1582 to fix the drift in the older Julian calendar. The reform skipped 10 days: October 4, 1582 was followed by October 15, 1582.
For modern date arithmetic, the ISO 8601 standard (published by the International Organization for Standardization) defines the date format as YYYY-MM-DD, week numbering rules, and representation of dates and times. We follow ISO 8601 throughout our date tools.
Leap Year Rules
The Gregorian calendar's leap year rules are:
- A year divisible by 4 is a leap year
- EXCEPT years divisible by 100 are NOT leap years
- EXCEPT years divisible by 400 ARE leap years
So 2000 was a leap year (divisible by 400), but 1900 was not (divisible by 100, not 400). The next leap years are 2028, 2032, 2036, 2040, and 2044.
Days in Each Month
| Month | Days | Notes |
|---|---|---|
| January | 31 | — |
| February | 28 or 29 | 29 in leap years |
| March | 31 | — |
| April | 30 | — |
| May | 31 | — |
| June | 30 | — |
| July | 31 | — |
| August | 31 | — |
| September | 30 | — |
| October | 31 | — |
| November | 30 | — |
| December | 31 | — |
Frequently Asked Questions
To calculate days between two dates, subtract the earlier date from the later date. For example, June 4, 2026 minus January 1, 2026 = 154 days. Our Days Between Dates Calculator handles this automatically, accounting for leap years and showing the result in days, weeks, months, and years. It also counts business days (excluding weekends) if needed.
Enter any date into our Day of the Week Calculator to find out which day it fell on. The algorithm uses the Zeller's congruence formula, the same one used by calendar applications worldwide. For example, July 4, 1776 was a Thursday, and January 1, 2000 was a Saturday. You can look up dates from year 1 to 9999.
To add days to a date, use our Date Calculator: enter the number of days, weeks, months, or years and the direction (from or before today). The result is computed using the proleptic Gregorian calendar. For example, 60 days from today is the date 60 calendar days ahead, accounting for varying month lengths and leap years.
A leap year has 366 days instead of 365, with an extra day (February 29) added to keep the calendar aligned with Earth's orbit. Leap years occur every 4 years, except for century years (which must be divisible by 400 to be leap years). So 2000 was a leap year but 1900 was not. The next leap years are 2028, 2032, 2036, and 2040.
To calculate your exact age, enter your birthdate into our Age Calculator. It shows your age in years, months, days, weeks, hours, minutes, and seconds — all updated in real time. For example, someone born on June 15, 1996 is currently 30 years, 0 months, and a number of days old.
A Unix timestamp is the number of seconds that have elapsed since January 1, 1970, 00:00:00 UTC (the Unix epoch). For example, timestamp 1700000000 corresponds to November 14, 2023. Unix timestamps are used by all major operating systems, file formats, and databases. Note: 32-bit systems will face the Year 2038 problem on January 19, 2038.
To convert 12-hour AM/PM time to 24-hour military time: for AM times (1:00 AM to 11:59 AM), remove AM and keep the same numbers (but 12:00 AM becomes 00:00). For PM times (1:00 PM to 11:59 PM), remove PM and add 12 to the hour (so 1:00 PM becomes 13:00, 11:00 PM becomes 23:00). 12:00 PM stays 12:00. Our 12-to-24-hour converter handles this automatically.
Calendar days include every day (Mon-Sun, 7 days per week). Business days (also called working days) include only Monday through Friday, excluding weekends. For most project planning, legal deadlines, and shipping estimates, business days are what matter. Our Business Days Calculator can add or subtract business days and optionally skip US federal holidays.
Real-World Applications
- Project management: Plan deadlines, sprints, and milestones with business-day awareness
- Personal: Countdown to birthdays, anniversaries, weddings, vacations, retirements
- Legal & contracts: Notice periods, statute of limitations, contract terms
- Health & medical: Due date calculations, medication schedules, recovery timelines
- Finance: Interest accrual periods, payment due dates, billing cycles
- Historical research: What day was the Battle of Hastings? What day did [event] happen?
- Software development: Convert Unix timestamps, debug date math, handle timezones
Authoritative Sources
Our date calculations are based on the following authoritative sources: