JLJosh Liljenquist

Date & time calculator

Date Calculator

Add or subtract calendar units from one date. Years and months are applied first with an explicit month-end rule, followed by weeks and days, so the result is reproducible and unaffected by the browser’s timezone.

By

Enter values and select Calculate.

How to use this calculator

  1. Choose a base date and Add or Subtract.
  2. Enter whole numbers for any combination of years, months, weeks, and days.
  3. Calculate to see a readable date, ISO date, and weekday.
  4. Use Days Between Dates when the job is comparing two dates instead.

Formula and method

Apply years, then months, then weeks, then days. If a year or month has no matching day, clamp to that month’s final valid date.

Worked example

January 31, 2026 plus one month becomes February 28, 2026. In leap year 2024, the same operation becomes February 29.

Assumptions and limits

  • The supported Gregorian year range is 1600 through 9999.
  • Multiple units are applied in this fixed order: years, months, weeks, days.
  • Month and year operations clamp to the final valid destination day.
  • A calendar date is not interpreted as a hidden local or UTC clock timestamp.

Understanding the result

The result follows the stated calendar or civil-clock rules rather than guessing a timezone, work policy, holiday calendar, or legal definition. Read the displayed endpoint and start-day policy before using a count in planning or records.

Common mistakes

  • Expecting January 31 plus one month to overflow into March
  • Applying days before months and expecting the same result
  • Entering a date outside the supported range
  • Using this tool to measure the distance between two dates

Methodology note

The calculator uses strict ISO-style date components, Gregorian leap-year rules, explicit clock values, and the deterministic policy shown on this page. It does not call a remote calculation, timezone, geolocation, payroll, calendar, or holiday service.

Need the number of days between two dates?Add or count business days

Browse all calculators.

This browser-local utility is a calculation aid, not legal, payroll, scheduling, employment, tax, or official-calendar advice. Confirm the rules that apply to your situation.