Time zones and daylight saving

1. What a time zone is

Earth turns once a day, so the sun rises at a different clock time depending on where you are. To keep clocks useful, the world is split into roughly 15-degree-wide bands (about an hour each) that share one local time, and that band is a time zone. The reference is UTC (Coordinated Universal Time), measured from the meridian through Greenwich, and each zone is described by its offset from UTC (Korea is UTC+9).

Computers and phones store these zones in the IANA Time Zone Database, a public table we read as-is and never edit by hand. When a country changes its time zone or daylight saving rule, this database is updated, and once a device installs that update its calculations follow the new rule automatically.

2. The basic rules

• A time zone is a political choice, not a strict function of geography, so countries on the same line of longitude can pick different zones.
• Most zones sit at a whole-hour offset from UTC, but not all: India uses UTC+5:30 and Nepal uses UTC+5:45.
• Daylight saving time moves clocks forward an hour in spring and back in autumn to shift daylight into the evening. Countries near the equator, where day length barely changes, mostly skip it.
• Some countries span several time zones (the US, Russia, Australia). This app shows one representative city (usually the capital) per country.

3. How this app calculates it

Sicha only stores a city name and its IANA time zone identifier (Seoul is Asia/Seoul). Every actual hour and offset is computed at that moment by the browser's `Intl`. We never hand-write an offset table, because such a table turns into a lie the instant daylight saving starts.

• The top city is the reference city. Dragging its slider (0-23) recalculates every other city's time at that same instant.
• A city on a different calendar day shows tomorrow or yesterday next to its time, a normal side effect of crossing midnight across zones.
• The chosen cities and slider value live only in the page address. Nothing is saved on a server and there is no account, so reopening the link shows the exact same screen.
• The daylight-saving dates on each country page (e.g. /en/country/usa) are found by scanning that time zone's offset day by day for the point where it changes.

4. Basic tips

• Add every city joining the meeting, then drag the slider until the hour works for everyone (roughly 9 a.m. to 6 p.m. local).
• Move your own city to the top to make it the reference. Remove it and add it back first.
• Once you settle on a time, copy the share link. Whoever opens it sees the same screen.

5. Advanced tips

• Recurring meetings scheduled near a daylight-saving transition (roughly March and October/November, dates vary by country) can shift by an hour without warning. Check the other country's time-difference page for this year's transition dates ahead of time.
• The slider compares hours for "today." To pin down a specific future date near a daylight-saving boundary, check each country's page separately for whether it will be observing daylight saving on that date.
• For half-hour or 45-minute zones (India, Nepal), pass along the exact minute shown rather than rounding to the hour.

6. Where people get stuck

• My city isn't in the list. Right now Sicha covers 25 representative cities. If your city shares a time zone with the listed one (true for most countries, less so for multi-zone ones like the US or Russia), the value is the same.
• A shared link shows a different time than I expected. The slider stores "what hour today" for that city. Opening the link the next day moves "today" forward, so the hour matches but the calendar date can differ.
• The time is off by an hour. Usually the device's time zone database is out of date and has not received a new daylight-saving rule yet. Updating the device's software usually fixes it.

Standard time difference by country (as of January 2026)

Add several cities on the home page to compare them all at once. Below is each country's standard difference from Korea (some shift during daylight saving, so check each country page for the exact dates).

Not a tracking tool

This app never asks for or stores your location. It only computes time zone data on screen. Source: IANA Time Zone Database (Public Domain), confirmed on 2026-09-06.