Time zone
A region's standard time as defined for each area of the world. It must be taken into account in international scheduling.
A time zone is a division of standard time adopted by each region of the world. Offsets from UTC are not only in 1-hour increments; some regions use 30-minute or 45-minute increments, and they are defined in the tz database (also called the Olson database) managed by IANA (Internet Assigned Numbers Authority). In scheduling, accounting for time zones is an essential part of setting up meetings for international teams.
Major time zones and UTC offsets
| IANA identifier | Abbreviation | UTC offset | Major cities |
|---|---|---|---|
| Asia/Tokyo | JST | +09:00 | Tokyo, Osaka |
| Asia/Seoul | KST | +09:00 | Seoul |
| America/New_York | EST/EDT | -05:00 / -04:00 | New York |
| Europe/London | GMT/BST | +00:00 / +01:00 | London |
| Asia/Kolkata | IST | +05:30 | Mumbai, Delhi |
Points to watch in scheduling
When scheduling for an international team, first identify the time periods when everyone’s working hours overlap. For example, the time difference between Tokyo (JST, +09:00) and New York (EST, -05:00) is 14 hours, and it becomes 13 hours while New York is on daylight saving time (EDT, -04:00). In either period, typical working hours (9:00-18:00) barely overlap, so one side has no choice but to join early in the morning or at night. In that case, a common approach is to rotate the meeting time so that the burden does not fall on one side.
In regions that use daylight saving time, the offset changes twice a year, so you also need to watch for recurring meetings suddenly shifting in time. If you set the start time in UTC, you can recalculate what time it falls on in each location each time, which makes shifts easier to notice. However, for recurring meetings that span a changeover date, it is more reliable to share in advance what time it will be in local time.
Use IANA identifiers, not abbreviations
A pitfall in practice is communicating time zones with abbreviations such as “JST” or “EST.” Abbreviations overlap (CST refers to different times in the central United States, China, and Cuba), and they cannot express daylight saving time changeovers. When handling events in calendars and systems, if you use the “Asia/Tokyo” or “America/New_York” format (IANA identifiers), the software handles everything automatically, including daylight saving time changeovers. In communication between people, adding a numeric offset such as “UTC+9” prevents misunderstandings.