Time Zones and Daylight Saving Time, Explained
Why the same meeting can move by an hour, and how to convert times without getting caught out by daylight saving.
What is a time zone offset?
What is a time zone offset? A time zone offset is the difference between local time and Coordinated Universal Time (UTC), written like UTC−5 or UTC+5:30. To convert between two places, compare their offsets on the specific date: New York at UTC−5 and London at UTC+0 are five hours apart. Offsets change when a region switches to or from daylight saving time.
UTC: the reference clock
Coordinated Universal Time is the world's reference time. It never changes for daylight saving, which makes it useful for scheduling across many regions. Every local time can be written as UTC plus or minus an offset.
Why abbreviations are unreliable
Abbreviations such as EST, CST and PST officially mean the standard (winter) time only. In summer the same regions use EDT, CDT and PDT. In everyday speech people use "EST" all year, so "3 PM EST" in July usually means 3 PM Eastern Daylight Time. Some abbreviations are also shared between countries: CST can mean US Central, China Standard Time or Cuba Standard Time.
Tools like Timer Point's converters use named regions from the IANA time zone database instead, such as America/New_York, which carry the full history of each region's rules.
How daylight saving affects conversions
Regions change their clocks on different dates, or not at all. The United States moves clocks forward on the second Sunday in March, while the European Union does so on the last Sunday in March. For a few weeks each spring and autumn, the gap between New York and London is four hours instead of five.
On the day clocks spring forward, one local hour does not exist; on the day they fall back, one hour happens twice. A careful converter warns you about both cases.
Practical tips for scheduling
A few habits prevent most mistakes:
- Include the date when converting, not just the time.
- Name a city or region rather than an abbreviation when inviting people.
- For recurring meetings across continents, double-check the weeks around March, April, October and November.
- When in doubt, state the time in UTC as well.