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Date math, done three ways.

Find the difference between any two dates, add or subtract a duration, or count business days, all in one tool. UTC arithmetic so leap years, month-end edge cases, and year-rollover never bite you.

How it worksReal-time

Inputs

Pick a mode

Days, weeks, months between two dates

Total days
—
Total weeks
—

Time between

Years · months · days

Pick two valid dates above to see the difference.

Field guide

Why date math is harder than it looks.

Date arithmetic looks simple — you subtract one date from another and you're done. In practice the calendar is a patchwork of compromises that has tripped up programmers for decades: months with different lengths, leap years, century rules, time zones, daylight saving transitions. This calculator avoids the worst traps by working in UTC and using calendar-aware arithmetic for years and months.

Mode 1: Difference between two dates

The most common question: “how many days from X to Y?” Subtracting milliseconds and dividing by 86,400,000 gives the day count. Translating that into years and months uses the standard calendar diff , exactly how you'd do it on paper.

totalDays = ⌊(toMs − fromMs) ⁄ 86,400,000⌋
totalWeeks = ⌊totalDays ⁄ 7⌋

Mode 2: Add or subtract a duration

For days and weeks, the math is millisecond addition. For months and years, you have to handle the calendar itself:

  • January 31 + 1 month → February 28 (or 29 in a leap year). The day clamps to the last valid day of the target month.
  • February 29 + 1 year → February 28 in non-leap years. Same clamping rule.
  • December 15 − 24 months → December 15 of two years prior. Years just shift.

The clamping rule is the same one most spreadsheets and databases use (Google Sheets EDATE, SQL DATE_ADD). It avoids the “Feb 31 became March 3” bug that naive implementations produce.

Mode 3: Business days

Counting business days means counting the Mondays through Fridays inclusive of both endpoints. The calculator walks the span day-by-day for short ranges and uses a closed-form formula (full weeks × 5 + remainder) for very long ones.

Holidays are not deducted automatically — they vary by country, region, and year. Take the workday count above and subtract the holidays that fall on weekdays in your jurisdiction. For example, US federal holidays in a typical year shave about 10 workdays off any full-year count.

Leap years

A year is a leap year if it's divisible by 4, except century years, which must also be divisible by 400. So 2000 was a leap year, 1900 wasn't, and 2100 won't be. Across a span of decades, leap years account for the gap between “365 × N” and the actual day count.

Worked example: contract renewal

A contract starts 2026-03-15 and runs for 18 months. When does it expire?

  • March + 18 months = September of the following year
  • Day 15 fits in September (which has 30 days)
  • Result: September 15, 2027

Worked example: project days

A sprint runs 2026-04-06 (Monday) to 2026-05-29 (Friday). How many working days?

  • Total calendar days (inclusive): 54
  • Full weeks: 54 ÷ 7 ≈ 7.7
  • Workdays: roughly 40 (8 Mon–Fri windows)

A note on time zones

This calculator treats both dates as plain calendar dates, without time-of-day or timezone semantics. That's the right behaviour for “how many days between June 1 and July 4” — what matters is the calendar difference, not what hour the day starts in your time zone.

Common uses

  • Contracts and leases: renewal dates, notice periods.
  • Project management: sprint length, deadline countdown.
  • Travel planning: trip duration, days until departure.
  • Pregnancy / due dates: weeks since LMP.
  • Subscriptions: when does the next billing fall on a weekend?