Free Online Date Calculator
Calculate date differences, add or subtract days, convert between formats, and calculate age — all in real time.
Key Features
Date Difference
Calculate days, weeks, months, and years between any two dates.
Date Arithmetic
Add or subtract days, weeks, months, and years from any date.
Format Conversion
Convert dates to ISO, US, European, and long-form string representations.
Age Calculator
Calculate exact age in years, months, and days from a birth date.
Frequently Asked Questions
General
What is the Date Calculator?
The Date Calculator is a multi-function date arithmetic tool with four modes: Difference (days/weeks/months/years between two dates), Add/Subtract (compute a date N days/weeks/months/years before or after a given date), Format (convert a date into ISO, US, European, and long-form string representations), and Age (calculate exact age in years, months, and days from a birth date). All modes update results in real time.
How to Use This Tool
- Select a mode tab: Difference, Add/Subtract, Format, or Age.
- Enter your date(s) using the native date picker or type in YYYY-MM-DD format. Click Today to jump to the current date.
- Results update automatically. Use the copy icon next to each output value to copy it to your clipboard.
Example: Date Difference
Input (Difference mode):
Start: 2025-01-01 End: 2026-01-01
Output:
365 days / 52 weeks / 12 months / 1 year
Details
How does the tool handle month arithmetic when the target day does not exist?
This is called "month overflow" and the tool uses end-of-month clamping. For example, adding 1 month to January 31 produces February 28 (or February 29 in a leap year), not March 3. Similarly, subtracting 1 month from March 31 gives February 28/29. This matches the behavior of JavaScript's Date constructor and most date libraries.
Does the age calculator account for leap years?
Yes. The age calculation accounts for the actual calendar days between the birth date and the reference date. This means someone born on February 29 will have their age correctly calculated.
What is the difference between the ISO date format and other formats?
ISO 8601 (YYYY-MM-DD) is the international standard for date representation and is the only format that sorts correctly as a string. Dates in YYYY-MM-DD format can be compared lexicographically without parsing. US format (MM/DD/YYYY) and European format (DD/MM/YYYY) are ambiguous with each other — which is why ISO is strongly preferred in all technical contexts.
How do I calculate the number of weekdays (business days) between two dates?
The current version counts all calendar days. To manually calculate business days, take the total day difference, subtract weekends (roughly 2/7 of the total), and subtract any holidays. A dedicated business day mode is planned for a future update.
Why does adding weeks sometimes produce a different result than adding the equivalent number of days?
Adding weeks and adding days should always produce the same result because a week is defined as exactly 7 days. If you see a discrepancy, it is likely due to daylight saving time (DST) transitions. Our "add weeks" mode uses calendar date arithmetic that preserves the clock time.