Time Zone Calculator

Convert date and time between countries, cities and time zones while accounting for UTC offsets and daylight saving time.

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About this Tool

Choose a source and destination city, enter the local time, and see the corresponding local time with its UTC offset.

How to use it

  1. Choose the country and city where your original time applies.
  2. Choose the destination and enter the date and time.
  3. Select Convert to see the destination time and offset.

Why people use this tool

  • Plan international meetings without mental timezone arithmetic.
  • Accounts for daylight-saving rules where the operating system provides them.
  • Shows the UTC moment so teams can confirm the conversion.
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 →

Related guides

Date & time 7 min read

Date Calculations Without the Off-by-One: D-Days, Anniversaries, Age and Months Old

How counting the first day changes D-day and anniversary results, how to calculate age in years, and how calendar-month ages and month-end dates work.

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 "Time Zone Calculator" that meets the requirements below.

## Goal

Build a tool that shows a date and time from one zone in several other time zones.

## Core features

- Enter a base zone, date and time
- Add or remove cities (IANA zones)
- Show UTC offset and whether DST applies
- Highlight overlapping office hours (09–18)
- Copy the result table

## Inputs

- Base zone, date and time
- List of zones to compare

## Outputs

- Local date and time per zone
- Offset from the base zone

## Edge cases and handling rules

- Clearly mark date changes (+1 day, −1 day)
- Offset changes around DST start and end
- Half- and quarter-hour offsets (India, Nepal)
- Use IANA names rather than fixed offsets

## 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)

- 2026-01-15 09:00 Asia/Seoul → 2026-01-14 19:00 America/New_York
- 2026-07-15 09:00 Asia/Seoul → 2026-07-14 20:00 America/New_York
- Asia/Kolkata is +05:30

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-timezone-calculator/SKILL.md for AI coding agents such as Claude Code to reuse whenever you build or change this tool.

---
name: build-timezone-calculator
description: Use when building or changing a Time Zone Calculator tool. Build a tool that shows a date and time from one zone in several other time zones.
---

# Build: Time Zone Calculator

## Goal

Build a tool that shows a date and time from one zone in several other time zones.

## 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

- Enter a base zone, date and time
- Add or remove cities (IANA zones)
- Show UTC offset and whether DST applies
- Highlight overlapping office hours (09–18)
- Copy the result table

## Inputs

- Base zone, date and time
- List of zones to compare

## Outputs

- Local date and time per zone
- Offset from the base zone

## Rules

- Clearly mark date changes (+1 day, −1 day)
- Offset changes around DST start and end
- Half- and quarter-hour offsets (India, Nepal)
- Use IANA names rather than fixed offsets
- 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

- [ ] 2026-01-15 09:00 Asia/Seoul → 2026-01-14 19:00 America/New_York
- [ ] 2026-07-15 09:00 Asia/Seoul → 2026-07-14 20:00 America/New_York
- [ ] Asia/Kolkata is +05:30

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.