Date Difference Calculator
How many days, weeks, months or years lie between two dates? Get the exact difference instantly — including a workdays count (Mon-Fri only). The second tab adds or subtracts days from a date.
Common Uses
- Project planning: exactly how many workdays until a deadline
- Tenure tracking: how long at a job, in a relationship, in a country
- Vacation planning: days until a trip, countdown to milestones
- Contract math: notice periods, lease end dates, statute of limitations
- Astronomy / history: days between historical events or astronomical phenomena
"30 days" vs "30 workdays"
A contract gives you "30 days to respond." Another gives "30 business days." Big difference:
| Type | Calendar Days | Real End Date (starting Mon, Jan 6, 2025) |
|---|---|---|
| 30 calendar days | 30 | Wed, Feb 5 |
| 30 business days | 42 | Mon, Feb 17 (skipping weekends + MLK Day) |
Roughly 30 business days = 42 calendar days = ×1.4 multiplier. Legal contracts almost always specify which. Always read the fine print.
Date Math Gotchas
- "One month from now" is ambiguous. Jan 31 + 1 month = Feb 28 (or 29). Mar 31 + 1 month = Apr 30. The "right" answer depends on convention; this calculator uses calendar-month arithmetic with day clamping.
- Leap years add a Feb 29 every 4 years — except century years not divisible by 400. So 1900 wasn't a leap year, but 2000 was. Next century test: 2100 (not a leap year).
- Time zones and daylight saving time can shift "days" calculations by an hour. This calculator uses local date only, no DST adjustment needed.
- Inclusive vs exclusive counting: "from Monday to Friday" — is that 4 days (Mon→Fri) or 5 days (counting both endpoints)? Different cultures and contexts use different conventions.
The Julian Day Number (Geeky Bonus)
Astronomers use the Julian Day Number system, which counts days continuously from January 1, 4713 BCE. Today's Julian Day Number is around 2,460,800. This system eliminates ambiguity about calendars and is used in scientific data and astronomy software. The US Naval Observatory's Julian Date Converter is the definitive reference.
Frequently Asked Questions
Why do I count differently if today is included?
"Day 1" can mean today (inclusive) or tomorrow (exclusive). This calculator uses standard exclusive counting — Mon to Tue = 1 day. For "from your birthday until now," that means age in completed full days.
How are workdays counted?
This calculator counts Monday through Friday as workdays, skipping Saturday and Sunday. It does NOT account for national holidays — for fully accurate business-day math, subtract your country's public holidays manually.
What about non-Gregorian calendars?
This calculator uses the Gregorian calendar (international standard since 1582). For Islamic, Hebrew, Hindu, or Buddhist calendars, conversion is more complex — try Fourmilab's calendar converter.