Time zones and daylight saving: how to get them right

UTC, offsets and time zones aren't the same thing

UTC is the world's reference time and never changes. An offset, such as UTC+1, is the difference from UTC at a particular moment. A time zone, such as Europe/London, is much more: a region's full set of rules, including past and future clock changes.

That's why storing “UTC+0” for a meeting in London is a mistake: in summer London is on UTC+1. The right approach is to store the zone (Europe/London) or the instant in UTC, and work out the local time when you display it.

Clock changes create times that don't exist and times that happen twice

In the European Union and the UK, on the last Sunday of March clocks jump forward an hour, so a slice of that night never happens. On the last Sunday of October the opposite occurs and an hour is lived twice.

Any program converting local times has to decide what to do in those cases. Our world clock warns you when you pick a time that doesn't exist or is ambiguous, instead of silently giving you a wrong answer.

Dates work best as calendar days

A classic bug is adding days by multiplying by 24 hours: if a clock change falls in between, the result shifts by an hour and, around midnight, can land on the previous day. This site's date tools work with calendar days, not 24-hour blocks, precisely to avoid it.

Timestamps: seconds or milliseconds

A Unix timestamp counts time since 1 January 1970 in UTC, so it has no time zone or daylight saving: it's the safest way to store an instant. The trap is the unit. Many systems use seconds (10 digits for current dates) while JavaScript uses milliseconds (13 digits). Mixing them up gives dates in 1970 or the year 50,000.

Tips for international meetings

• Always state the time zone with the time (“5 pm London time”), never just an offset.
• Be wary of abbreviations: CST can mean US Central, China Standard or Cuba Standard Time.
• Double-check the weeks in March–April and October–November: countries change clocks on different dates, so the gap between cities varies temporarily.
• Use a converter with IANA zones to check the time in every city at once.