In more detail
UTC (Coordinated Universal Time) is kept by atomic clocks and occasionally adjusted with leap seconds to stay close to the Earth's rotation. A zone such as +02:00 is two hours ahead of UTC; real offsets run from −12:00 to +14:00, and a few are not whole hours, such as India at +05:30 and Nepal at +05:45. UTC itself never changes for daylight saving. The letter Z in 14:30:05Z means UTC, which is why it is sometimes called "Zulu time".
Why it matters
Storing times in UTC and converting only for display avoids most date bugs, such as events shifting by an hour when clocks change. An offset is not a time zone, though: Madrid is +01:00 in winter and +02:00 in summer, so future events in local time should be stored with a named zone such as Europe/Madrid. Logs from servers in different places can only be compared reliably in UTC. See How Dates and Time Zones Work.
Example
A meeting at 14:30 UTC on a summer date is:
- 16:30 in Madrid (CEST, +02:00)
- 10:30 in New York (EDT, −04:00)
- 20:00 in Mumbai (IST, +05:30)
Common confusion
GMT is a time zone used by the UK in winter and shows the same clock time as UTC; UTC is the scientific standard, and the UK switches to +01:00 in summer. In ISO 8601 timestamps, UTC is written Z or +00:00.