World Time Grid
Compare one moment across cities in a grid and see daylight spread across the world map.
How to use it
- Search for cities to add comparison columns, then use the arrows on each city card to reorder them.
- Open + Add a time, choose the reference city, and enter its local date and clock time. Every row represents one simultaneous instant worldwide.
- Select a row to update the day/night map. Use Share link only when you want to copy the current cities and moments into the URL.
Examples
Check a New York morning meeting from Seoul
Enter 2026-06-12 09:00 with New York as the reference. The same instant appears as 22:00 in Seoul and 14:00 in London.
- Input
New York · 2026-06-12 09:00
- Result
Seoul 22:00 · London 14:00
Compare across the date line
Put Honolulu and Auckland side by side to reveal when the same instant falls on different calendar dates, marked as +1 day or −1 day.
Details
World Time Grid uses the browser’s Intl.DateTimeFormat and built-in time-zone database. UTC offsets are calculated for each selected instant rather than treated as fixed numbers, so daylight-saving time and historical rule changes are reflected as far as the browser’s data allows. When a local wall time must be converted back to an instant, the tool checks possible offsets and verifies the round trip. A skipped spring-forward time is rejected. A repeated fall-back time defaults to its earlier occurrence, with an explicit option for the later one.
The grid classifies ● work as 09:00–17:59, ◐ evening as 18:00–21:59, and ○ night as all other local clock times. These are simple planning cues, not claims about actual working hours or sunrise. The map shadow is calculated from solar declination and the subsolar point for the selected instant; atmospheric refraction and twilight are not modeled. Very recent legal time-zone changes may appear only after your operating system or browser receives an updated time-zone database.
Frequently asked questions
What happens if I enter a time skipped by daylight saving?
A time such as 02:30 on New York’s spring transition does not exist, so the tool warns you and does not add a row.
Which occurrence is used when a fall-back time repeats?
The earlier instant is the default. Choose the later option in the add-time panel when you need the second occurrence.
Are my cities and times saved automatically?
No. Nothing is written to browser storage or a server. The state enters the URL hash only after you press Share link.
Why can a night-marked cell sit on the daylight side of the map?
Cell symbols represent broad clock-based activity bands. Map shading represents astronomical day and night, which vary by season and latitude.