About Date Diff Calculator
Date Diff Calculator returns two views of the same span. The calendar breakdown borrows month-by-month the way a person counts ages and anniversaries. The totals divide the raw millisecond difference into weeks, days, business days, hours, minutes, and seconds.
Common uses: age calculation, project deadlines, contract terms, billing periods.
How to Use
- Pick a start date and time— The "Now" button drops in the current moment.
- Pick an end date and time — Order does not matter; the result flags when end precedes start.
- Read the breakdown — Years/months/days are calculated using calendar borrowing so the result reflects how humans count duration.
- Use Swap — Quickly invert the two inputs.
Frequently Asked Questions
Q: How do you count business days?
Business days are Monday through Friday between the two date components (ignoring the time portion). National or regional holidays are not subtracted.
Q: Why are months and days computed differently from total days?
Calendar months have different lengths (28-31 days), so a year-month-day breakdown borrows from the actual previous month. The total-days view simply divides the raw millisecond difference, which can produce slightly different intuition for periods that span uneven months.
Q: Are time zones handled?
The inputs use your local time zone. If you need a fixed offset or a specific city, use our Timezone Converter to align values first.
Q: What is the maximum date range?
Limited only by the JavaScript Date type, which covers ±100,000,000 days from Jan 1, 1970 — far beyond any practical use.
Q: Does the end-before-start case work?
Yes — the magnitude is still calculated and a label warns you that the end is earlier than the start.
Timestamp reference
Common timestamp representations and when each applies.
- Unix seconds — integer, 10 digits circa 2026. Most Postgres / Redis / log timestamps.
- Unix milliseconds — 13 digits. JavaScript
Date.now(), MongoDB, Kafka. - ISO 8601 —
2026-04-24T10:30:00Z. JSON APIs, logs, Kubernetes. - RFC 2822 —
Fri, 24 Apr 2026 10:30:00 GMT. Email, HTTP Date header. - Windows FILETIME — 100-ns intervals since 1601-01-01. NTFS, some SMB protocols.