Date calculator
Ten date modes in one tool: gap between dates, D-day, D+, add or subtract days, weekday, months apart, age, days lived and Mon-Fri working days.
What to know
The date calculator is one form that changes shape with the mode. Two-date modes show Start date and End date, offset modes show a start date plus a Days field, and birth-date modes show a single Birth date field.
All arithmetic is done on whole calendar days in UTC, so daylight saving never adds or removes an hour from a result. A gap is end minus start, which means 2025-01-01 to 2025-01-02 is 1 day and the starting day itself is not counted.
The modes are date difference, D-day, D+ day, date after N days, date before N days, weekday, month difference, age, days lived and a working-day counter. The working-day mode counts Monday to Friday on both selected dates and ignores public holidays.
Fields marked as dates reject impossible values such as 2025-02-30, and the Days field accepts whole numbers of 0 or more. D-day, D+, age and days lived take today from your device clock, so the result moves when the date changes.
Two details keep the modes honest. Impossible dates such as 2025-02-30 are rejected before any arithmetic runs, and each result line carries its own label, for example Days since start or Working days (Mon-Fri), so a copied number is never ambiguous about what it measures.
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How to use Date calculator
- Open the mode that matches the question, for example date difference.
- Type or pick Start date 2025-01-01 and End date 2025-12-25.
- Read the Result line, which shows 358 days for that pair.
- For an offset mode, enter a whole number such as 100 in the Days field instead of an end date.
- Use Reset to put today in the date fields again.
Quick answers
Does a date difference include the first day?
No. The result is end minus start, so 2025-01-01 to 2025-01-31 returns 30 days. Add 1 yourself if you need to count both ends.
Do the working-day results skip public holidays?
No. The working-day mode counts Monday to Friday, including both selected dates, and has no holiday calendar.
Why did my D-day result change overnight?
D-day, D+, age and days lived compare against the date on your device. When the device date rolls over, the number moves by one.
Can I enter a negative number of days?
No. The Days field must be a whole number of 0 or more; use the date-before page to go backwards.