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Date, Time & Planning

Date and Time Simulation Tool

Shift a base date and time forward or backward in any IANA zone, then export the simulated instant and a bounded edge-case fixture set for date shift testing.

LocalNo input retention

Reviewed September 11, 2026

Guide, examples, and validation Show

About this tool

Generate the shifted date and time values a forward-date test needs, without touching a system clock.

  • Shifts a base date and time by minutes, hours, days, weeks, months, or years in any IANA zone this browser recognizes, then optionally snaps the result to the start of next month, end of month, end of quarter, end of year, the next leap day, the next DST transition, or the same time tomorrow.
  • Outputs the simulated instant as ISO 8601 with offset, ISO 8601 UTC, Unix epoch seconds and milliseconds, RFC 2822, HTTP-date, a local formatted string, SQL DATETIME, and the day of week, each with its own copy button.
  • Builds an edge-case fixture table covering month-end lengths of 28, 29, 30, and 31 days, leap day, quarter ends, the year rollover, daylight-saving spring-forward and fall-back instants, plus 14, 30, 90, and 365-day expiry boundaries, and exports it as JSON or CSV alongside Jest, freezegun, and java.time mock setup snippets.

How to use Date Simulator

Set the base date and time, choose the IANA zone, enter a signed shift amount and unit, optionally pick a snap preset, then run the simulation and review the notes before copying a value, exporting the fixture set, or downloading the JSON evidence report.

When this tool is useful

  • Preparing forward-date test values for month-end, quarter-end, year rollover, leap day, or expiry scenarios.
  • Finding the exact daylight-saving spring-forward and fall-back instants for a specific zone before writing a regression test.

Practical tips

  • Month and year shifts clamp to the last real day of the target month, so read the clamp note rather than assuming Jan 31 plus one month is Mar 2.
  • Day and week shifts keep the local wall-clock time while minute and hour shifts add exact elapsed time; pick the one that matches what your code actually measures.
  • Run a Jest or freezegun suite with the same TZ as the zone selected here, because freezing the clock does not change the process time zone.

Examples you can test

Load an example, compare the result with the expected output, then replace it with your own input.

Shift a billing date past month end

Example input

2026-01-31 09:00 in UTC, plus 1 month

Expected output

2026-02-28T09:00:00+00:00 with an explicit month-end clamp note, plus a fixture table and mock snippets

The value is generated for your test harness; no system clock is changed.

Validation checklist

  • Confirm the selected IANA zone matches the zone the application under test uses.
  • Read every note, especially month-end clamping and skipped or repeated wall-clock times around DST transitions.
  • Verify the simulated behavior inside your own application, because generated values prove arithmetic rather than application correctness.

Frequently asked questions

Can this change my computer's system clock?

No. A browser page cannot change the system clock of your machine, container, or CI runner. Desktop date shifting software such as a system time machine or RunAsDate does that at the operating-system level. This tool produces the date and time values and fixtures you feed into mocks, seed data, configuration, or test parameters.

How do I simulate a future date in my app?

Run the date shift here, copy the simulated ISO 8601 instant, then inject it through a clock your code already reads: jest.useFakeTimers().setSystemTime() in JavaScript, freezegun's freeze_time in Python, or an injected Clock.fixed in Java. The Mock setup code tab renders all three with the simulated instant and the selected zone filled in.

How are month-end and leap-day shifts handled?

Month and year shifts clamp to the last real day of the target month, so January 31 plus one month is February 28 in a common year and February 29 in a leap year, and the clamp is reported as an explicit note. Minute and hour shifts add exact elapsed time, while day and week shifts move the calendar date and keep the same local wall-clock time.

How do I find DST transition dates for testing?

Select the zone and run the simulation. The fixture table includes the spring-forward and fall-back instants for that zone, one second before and at each transition, and the snap preset can jump straight to the next transition. If the zone does not observe daylight saving within the scanned window, the tool says so instead of inventing a date.

Which formats are exported, and is anything uploaded?

Nothing is uploaded. Every calculation runs in this browser. The simulated instant is available as ISO 8601 with offset, ISO 8601 UTC, epoch seconds and milliseconds, RFC 2822, HTTP-date, a local formatted string, SQL DATETIME, and the day of week, and the fixture set downloads as JSON or CSV with a versioned JSON evidence report.

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