Date Calculator

Difference between dates and date arithmetic

Difference between dates

0
days
0
weeks
0
months (approx)
0
years (approx)

About

Compute the number of days between two dates, or add and subtract days, weeks, months and years from a starting date. Useful for deadlines, billing cycles and project planning. Runs entirely in your browser.

    Examples

    Compute days between two dates

    Input
    from: 2024-01-15
to: 2024-03-20
    Output
    65 days = 9 weeks 2 days = 2 months 5 days

    Day count is exact; week and month counts use calendar-aware subtraction so '2 months' lines up with the 15th of March, not a fixed 60-day window.

    Add 90 business days to a date

    Input
    start: 2024-06-01
add: 90 business days (skip Sat/Sun)
    Output
    2024-10-08

    Skips weekends so the result lands on a weekday, useful for projecting delivery or SLA deadlines.

    About this tool

    The Date Calculator does two complementary things: computes the difference between two dates in days, weeks, months and years, and adds or subtracts a duration to a starting date. Both operations are calendar-aware, so day counts are exact and month/year arithmetic lands on a real calendar date (Jan 31 + 1 month = Feb 28, not Mar 3).

    It respects your local timezone and DST rules, and can switch to UTC for stable, timezone-independent intervals. All math runs in your browser; your dates are not uploaded or stored.

    How to use

    1. Pick a mode

      Choose Difference to find the interval between two dates, or Add/Subtract to project a date forward or backward by a chosen duration.

    2. Enter the dates or duration

      For Difference, set the start and end dates. For Add/Subtract, set a start date and a duration in days, weeks, months or years.

    3. Read the result

      The tool shows the interval in calendar units or the projected date. Switch the timezone to UTC for a stable, region-independent result.

    Use cases

    Calculating SLA and trial windows

    Compute 'N days from now' or 'days until expiry' for support tickets, free trials, and license terms without doing calendar math by hand.

    Projecting timelines

    Add a fixed number of weeks or months to a project start date and see the resulting calendar date, accounting for short months and leap years.

    Verifying a colleague's date arithmetic

    Sanity-check a date diff you computed inline in a spreadsheet before quoting it in a contract or report.

    Common mistakes

    Mistake:Treating a 'month' as 30 days.

    Fix:Calendar months vary from 28 to 31 days. The calculator uses calendar-aware subtraction so Jan 31 + 1 month = Feb 28/29, not Mar 3.

    Mistake:Ignoring the user's timezone.

    Fix:Two dates with the same calendar date in different timezones can be 24 hours apart in absolute time. Switch to UTC if you need a stable, tz-independent result.

    Mistake:Forgetting DST transitions.

    Fix:Spring-forward and fall-back days have 23 or 25 hours. The calculator respects local DST, so '24 hours from now' is always exactly 24 hours even across a transition.

    Frequently asked questions

    References & standards

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