Date & Time Calculator

Date calculator for differences and date math

This date calculator answers two everyday questions: how much time lies between two dates, and what date you reach after adding or subtracting a period. It works with plain dates or exact date-times and reports the answer in calendar units as well as totals.

Difference between two dates

Pick a start and an end date, in either order. The calculator sorts them, notes whether the end is before or after the start, and shows the gap:

Start date:     2024-01-15 00:00:00
End date:       2024-12-25 18:30:00

Difference:     11 months, 10 days, 18 hours, 30 minutes
Total weeks:    49.40
Total days:     345.77
Total hours:    8,298.5
Business days:  247

Total minutes and seconds are listed too. Business days count Monday to Friday, including the start date but not the end date; public holidays are not known to the tool, so subtract them yourself.

Adding and subtracting time

Pick add or subtract, an amount and a unit: seconds, minutes, hours, days, weeks, months or years. The result is shown in the selected time zone and in UTC, with its weekday and Unix timestamp. For example, 2024-01-15 plus 90 days is Sunday, April 14, 2024.

How month-end dates behave

Months and years are calendar units, not fixed lengths. When the target month is shorter, the day is clamped to its last day:

2024-01-31 + 1 month  = 2024-02-29   (leap year)
2023-01-31 + 1 month  = 2023-02-28
2024-03-31 − 1 month  = 2024-02-29
2024-02-29 + 1 year   = 2025-02-28

Clamping means steps do not always combine. 2024-01-31 plus 2 months is March 31, but adding 1 month twice gives March 29, because the first step lands on February 29. Month and year amounts should be whole numbers; a fractional amount such as 1.5 months is truncated to 1.

Smaller units are exact durations and may be fractional: 2.5 days adds 60 hours.

Time zones and daylight saving

Dates are picked as clock times in the time zone selected at the top, which starts on your browser's own zone; choose UTC or any other zone from the list. A difference is measured between the two clock readings, so a daylight saving change in between does not add or remove an hour: 12:00 one day to 12:00 the next is always exactly 1 day. When adding or subtracting, seconds to weeks are exact durations, so one day is always 24 hours and the clock time moves by an hour when the span crosses a daylight saving change, while months and years keep the clock time. Select UTC to avoid such shifts.

Typical uses

• Deadlines and notice periods: 30 days from a contract date.
• Ages and anniversaries in years, months and days.
• Project planning with business-day counts.
• Checking token lifetimes or cache expiry in hours or seconds.

To turn a result into epoch time or view it in another zone, use the Unix timestamp converter. For tallying hours worked, the time sheet calculator is a better fit, and the time converter changes one unit into another.