Date calculator

How to calculate the difference between dates

1. Enter the start date and end date using the date pickers.
2. The tool shows the difference in days, with weeks and approximate months and years.
3. Alternatively, enter a date and add or subtract a number of days to find a future or past date.
4. All calculations happen instantly in your browser.

Common uses

• Calculating how many days until an event or deadline
• Working out someone's age or how long ago something happened. Dates from APIs often arrive as epoch numbers, so turn the Unix timestamp into a date first.
• Adding days to a date for scheduling or deadlines. For a birthday countdown with the exact age, open the age calculator.

Leap years, ISO 8601 dates and daylight saving are explained in the guide to dates, timestamps and time zones.

Technical specification

• Algorithm or formula: Difference mode subtracts two epoch millisecond values (both dates at local noon) and rounds to whole days. Weeks are days divided by 7, and months and years are approximations (days / 30.4375 and days / 365.25). Add or subtract mode calls setDate on a local-noon Date. Age counts whole calendar months, then the days since the last monthly anniversary.
• Browser API or library: Native JavaScript Date. No external library.
• Input limits: Any date the browser's date picker accepts, from year 1. Add or subtract takes a whole number of days; a result before year 1 or beyond the JavaScript Date range is not shown.
• Output: Days between two dates with weeks and approximate months and years; the new date as a long local date (for example 29 October 2026); age in years, months, days and total days lived; days and weeks until or since a date.
• Known limitations: Every calendar day counts, so there is no business-day or holiday handling. The end date is not included in the count (add 1 to include it). Calculations use the browser's local time zone.

Frequently asked questions

How many days are between two dates?

Pick a start and an end date and the tool subtracts them: 1 January 2026 to 1 March 2026 is 59 days, or 8 weeks and 3 days. The end date is not counted, so add 1 to include both dates.

What date calculations does this tool perform?

Four modes: days between two dates (with weeks and approximate months and years), add or subtract a number of days from a date, exact age from a date of birth, and days until a date. All of them use the browser's built-in Date, which handles leap years and month lengths.

Is my data sent to a server?

All date calculations run in your browser using JavaScript's built-in date functions. Our servers are not involved at any point.

Does the tool account for leap years?

Yes. The JavaScript Date API handles leap years correctly in all difference and addition calculations.

Can I calculate business days (excluding weekends)?

Every count includes all calendar days, so weekends and holidays are counted too. The tool has no business-day option.

Can I add months or years to a date, not just days?

The add/subtract mode takes a number of days. Use a negative number to go back in time. To add a month, enter the number of days in that month.