Guide
Terminology for civil time, with links into the working desk.
01
Coordinated Universal Time is the civil time scale used as a reference. Offsets are stated relative to UTC.
Example: 15:00 UTC on 2026-11-06 is a single instant, independent of city names.
Misunderstanding: Treating UTC as a city that observes summer time.
02
Greenwich Mean Time is a time scale historically tied to the Greenwich meridian. In casual speech people mix GMT and UTC; they are not interchangeable in every historical or technical context.
Example: Europe/London may equal GMT in winter and BST in summer.
Misunderstanding: Labeling a July London time as GMT+0.
03
Identifiers such as America/New_York name a region’s rule set, including historical and seasonal offsets.
Example: Asia/Kolkata stays on UTC+05:30 in current rules.
Misunderstanding: Using EST as if it uniquely meant New York.
04
An offset is the difference between local civil time and UTC at a given instant. It can include 30- or 45-minute components.
Example: Australia/Adelaide is not a whole number of hours.
Misunderstanding: Adding five hours to New York all year to get London.
05
Some regions change civil offsets seasonally. Others never do. Transition dates differ.
Example: Phoenix uses America/Phoenix and does not follow US federal DST in current rules.
Misunderstanding: Assuming every country changes clocks on the second Sunday in March.
06
The date line is a civil convention in the Pacific. Crossing it changes the calendar date without requiring a 24-hour elapsed wait.
Example: Auckland can be a calendar day ahead of Honolulu at the same instant.
Misunderstanding: Thinking the date line is a single straight 180° meridian with no exceptions.
07
CST, IST, and similar abbreviations are reused in more than one region. Prefer IANA identifiers in calculations.
Example: IST may mean India or Irish Standard Time depending on context.
Misunderstanding: Sending “Let’s meet at 9 AM CST” without a city.
08
When clocks move backward, a local time can occur twice. When they move forward, a local time can be skipped.
Example: 01:30 on a US fall-back morning may be either offset.
Misunderstanding: Silently picking one occurrence without telling the reader.
01
Incorrect assumption: New York is UTC−5
Why it can fail: Seasonal rules change the offset.
Correct approach: Resolve the named zone on the meeting date.
Practical example: January vs March New York–London differences.
02
Incorrect assumption: 18:00 here is 18:00 there on the same date
Why it can fail: The instant can fall on the previous or next local date.
Correct approach: Always print local date and time.
Practical example: New York evening to Sydney morning.
03
Incorrect assumption: 9:00 must be morning
Why it can fail: A 12-hour clock is easy to misread in writing.
Correct approach: Use 24-hour time or spell out morning/evening.
Practical example: 21:00 written as 9:00.
04
Incorrect assumption: CST is enough
Why it can fail: Several regions share the letters.
Correct approach: Use America/Chicago or a city name plus offset.
Practical example: A message that could mean China or Central US.
05
Incorrect assumption: Last month’s offset still applies
Why it can fail: A transition week can change who is ahead.
Correct approach: Check the DST checker for that year.
Practical example: US and UK start dates that do not match.
06
Incorrect assumption: Everyone springs forward
Why it can fail: Many zones never change.
Correct approach: Look up the IANA zone instead of a country stereotype.
Practical example: Tokyo in June vs London in June.
07
Incorrect assumption: A two-hour meeting ending at start plus two local hours
Why it can fail: A DST skip or repeat changes wall-clock arithmetic.
Correct approach: Add duration to the instant, then convert.
Practical example: A meeting that starts before a spring-forward.
08
Incorrect assumption: 10:00 organizer time is fine for everyone
Why it can fail: It may be 23:00 elsewhere.
Correct approach: Compare working-hour preferences first.
Practical example: New York 09:00 with Tokyo on the same instant.
Not every zone uses daylight saving time, and not every offset change is seasonal. This checker reports transitions present in the installed database for the selected year.