Unix Timestamp Converter

Convert date/time values to Unix seconds, milliseconds, and ISO strings.

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// about

About this Tool

Computers count time in total seconds starting from January 1, 1970 (called Unix Timestamp). This tool turns those big numbers into real dates and times, or turns dates into timestamp numbers.

How to use it

  1. Paste a timestamp number OR choose a calendar date.
  2. Select whether your number is in seconds or milliseconds.
  3. Click 'Convert' to see the converted date and time result.

Why people use this tool

  • Translates computer numbers like 1700000000 into human dates instantly.
  • Supports both seconds and milliseconds format.
  • Great for checking log files, server times, and software bug reports.

Frequently Asked Questions

What is Unix Timestamp?

It is a simple number counting total seconds passed since midnight UTC on Jan 1, 1970.

Why are some timestamps 10 digits and others 13 digits?

10-digit numbers count seconds, while 13-digit numbers count milliseconds.

What timezone is displayed?

Results show standard UTC time as well as your local time zone.

Guides · Date & time · 7 min read

Unix Timestamps Explained: Seconds vs Milliseconds, Time Zones and the Year 2038 Problem

When is 1700000000? Why do some systems use 13-digit numbers? Why is the time off by exactly nine hours? Most time bugs disappear once you understand a few concepts.

A Unix timestamp (Unix time, epoch time) is a single integer counting the seconds elapsed since 00:00:00 UTC on 1 January 1970, the Unix epoch. 0 is 1970-01-01 00:00:00 UTC and 1700000000 is 2023-11-14 22:13:20 UTC.

0           → 1970-01-01 00:00:00 UTC
1700000000  → 2023-11-14 22:13:20 UTC
1735689600  → 2025-01-01 00:00:00 UTC
2147483647  → 2038-01-19 03:14:07 UTC
What a Unix timestamp is, how to tell 10-digit from 13-digit values, converting between UTC and local time, daylight saving pitfalls and the 2038 problem.
Read guide →

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What each of the five cron fields means, how to use *, /, - and commas, common schedules, and the pitfalls around day fields and time zones.

Read guide
Build it with AI

Build this tool yourself with AI

Want your own version of this tool? Copy the work-order prompt into an AI chat, or save the skill file for an AI coding agent. Both describe the features, edge cases and test cases we used when building it.

AI work-order prompt

Paste it as-is into an AI chat such as Claude, ChatGPT or Gemini.

You are an experienced full-stack web developer. Build a web tool called "Unix Timestamp Converter" that meets the requirements below.

## Goal

Build a tool that converts between Unix timestamps and human-readable dates.

## Core features

- Timestamp → date and date → timestamp
- Auto-detect seconds (10 digits) or milliseconds (13 digits)
- Show UTC, local and a chosen IANA zone together
- ISO 8601 output
- 'Now' button

## Inputs

- A timestamp or a date and time
- Time zone

## Outputs

- Converted date or timestamp
- ISO 8601 string
- Relative time (e.g. 3 days ago)

## Edge cases and handling rules

- Support negative timestamps before 1970
- Handle values after 2038 correctly
- Explain skipped or repeated local times at DST transitions
- Validate non-numeric input

## Implementation requirements

- Build it as a single page (HTML, CSS, JavaScript) that runs entirely in the browser, and never send the user's input to a server.
- Make the UI responsive and easy to use on mobile; label every input and keep everything keyboard accessible.
- Add a Copy button for results and show clear, friendly error messages for invalid input.
- Keep the core logic in functions separate from the UI so it can be unit-tested.

## Definition of done (test cases)

- 0 → 1970-01-01T00:00:00Z
- 1700000000 → 2023-11-14T22:13:20Z
- 1700000000000 (13 digits) is the same instant

First outline your implementation plan in at most five lines, then write the complete code, and finally explain how to check the test cases above.

Skill (SKILL.md)

Save it as .claude/skills/build-timestamp-converter/SKILL.md for AI coding agents such as Claude Code to reuse whenever you build or change this tool.

---
name: build-timestamp-converter
description: Use when building or changing a Unix Timestamp Converter tool. Build a tool that converts between Unix timestamps and human-readable dates.
---

# Build: Unix Timestamp Converter

## Goal

Build a tool that converts between Unix timestamps and human-readable dates.

## Workflow

1. Review the required features, inputs and outputs below and ask about anything missing first.
2. Implement the core logic as pure functions separate from the UI.
3. Check the logic against the test cases in the verification list.
4. Build a responsive UI with an input form, a result area and a Copy button.
5. Reproduce each edge case listed under Rules and refine the error messages.

## Core features

- Timestamp → date and date → timestamp
- Auto-detect seconds (10 digits) or milliseconds (13 digits)
- Show UTC, local and a chosen IANA zone together
- ISO 8601 output
- 'Now' button

## Inputs

- A timestamp or a date and time
- Time zone

## Outputs

- Converted date or timestamp
- ISO 8601 string
- Relative time (e.g. 3 days ago)

## Rules

- Support negative timestamps before 1970
- Handle values after 2038 correctly
- Explain skipped or repeated local times at DST transitions
- Validate non-numeric input
- Build it as a single page (HTML, CSS, JavaScript) that runs entirely in the browser, and never send the user's input to a server.

## Verification

- [ ] 0 → 1970-01-01T00:00:00Z
- [ ] 1700000000 → 2023-11-14T22:13:20Z
- [ ] 1700000000000 (13 digits) is the same instant

Tips for working with AI

  • Tell the AI your stack (for example React, Vue or plain HTML) on the first line so the code matches your project.
  • Ask it to run the listed test cases and show the results before you accept the code.
  • Never paste real secrets, customer data or API keys into the prompt; use sample values instead.