Date Calculator

Runs in browser Utilities

Compute future or past dates from a start date, with month-end awareness and business-day mode, and see the result in ISO, local and UTC formats.

Privacy: This tool runs entirely in your browser. Your input never leaves your device.

What is an ISO 8601 timestamp, and how does it relate to Unix time?

ISO 8601 writes a moment as 2026-09-23T10:15:00+03:00 — largest unit first, with an explicit UTC offset or Z — so it sorts as text and is unambiguous across locales; RFC 3339 is the Internet profile APIs use. Unix time is the same moment as a count of seconds (or milliseconds in JavaScript) since 1970-01-01T00:00:00Z, with no time zone at all. Most date bugs are an offset applied twice or a local time stored as if it were UTC.

How to use Date Calculator

  1. Enter the start date (or "now").
  2. Choose Add or Subtract and fill in the amounts.
  3. Read the result and the step-by-step table.

Date Calculator features

  • Add or subtract years, months, weeks, days, hours, minutes and business days
  • Month-end clamping (31 Jan + 1 month = 28/29 Feb) with an explanatory note
  • Configurable weekend for business-day arithmetic
  • Step-by-step table showing the intermediate dates
  • Result in local, ISO, UTC, RFC 2822 and epoch formats

Date Calculator example

Invoice due date: 30 business days

Input:

2024-03-15 + 30 business days (weekend Fri–Sat)

Output:

2024-04-25 (Thursday)

Frequently asked questions about Date Calculator

What happens when I add one month to 31 January?

The day is clamped to the last valid day of the target month, so you get 28 (or 29) February. The step-by-step table notes every time this clamping happens.

In which order are the units applied?

Years, then months, weeks, days, business days, hours and finally minutes — the same order as most calendar libraries, which keeps results predictable.

How are business days added?

The calculator moves one calendar day at a time and only counts days that are not part of the selected weekend, so "+10 business days" from a Monday lands two weeks later.

Does daylight saving time affect the result?

Calendar units (days, months) keep the same wall-clock time across a DST change; hours and minutes are exact elapsed time. Choose the timezone that matches your use case.

References