Guide
How to Add or Subtract Days and Find Date Differences
Learn how date maths works across months and leap years, and how to find the days between two dates.
A date calculator adds or subtracts years, months, weeks, or days from a start date, or measures the exact difference between two dates. It handles the calendar irregularities that make mental date maths error-prone.
How date maths works
Adding months or years to a date is not always straightforward: 31 January plus one month lands on 28 February (or 29 in a leap year), not 3 March, because February has fewer days. A date calculator applies calendar rules so the result is a valid date.
Finding the difference between two dates
The calculator counts the whole days between the start and end dates, inclusive or exclusive depending on convention. It can also break the difference into years, months, and days for human-readable durations.
How to use the calculator
- Enter a start date.
- To add/subtract: choose the units and amount, then read the result date.
- To find a difference: enter the end date and read the days (or years, months, days) between.
Examples
Adding 45 days to 1 January 2024 gives 15 February 2024. The difference between 1 January 2024 and 1 March 2024 is 60 days (2024 is a leap year). Adding one month to 31 January 2024 gives 29 February 2024.
When a date calculator is useful
- Planning deadlines, projects, and renewals.
- Calculating contract or subscription durations.
- Counting days until an event or trip.
- Working out age or tenure from dates.
Put it into practice
Use the Date Difference Calculator right now — free, in your browser, no sign-up required.
Try the Date Difference CalculatorMore guides
- How to Unscramble Letters Into Words
- How to Find Anagrams of Any Word
- How to Count Words, Characters and Reading Time Accurately
- How to Find Words by Pattern, Length and Letters
- How to Use a Random Word Generator for Games, Writing and Brainstorming
- How to Count Characters (With and Without Spaces)
- How to Convert Text Between Cases (UPPER, lower, Title, camelCase and More)
- How to Clean Up Messy Text (Spaces, Line Breaks and Whitespace)
Related free tools
More free online utilities that pair well with this one — same-category tools for the same task, plus handy extras from other categories.