A UTC offset is a signed number that expresses how far ahead of or behind Coordinated Universal Time (UTC) a given location is at a given moment. UTC+5:30 means five hours and thirty minutes ahead of UTC. UTC−8 means eight hours behind.
Offsets are more reliable than timezone names for one specific reason: they are fixed values. UTC+5:30 always means UTC+5:30. "India Standard Time" also always means UTC+5:30 — but "Eastern Time" means UTC−5 in winter and UTC−4 in summer, because Eastern Time observes DST. The name is stable. The offset it refers to is not.
The problem with named timezones
"Eastern Time" is a label that maps to two different offsets depending on the date: EST (UTC−5) in winter and EDT (UTC−4) in summer. "Central European Time" maps to UTC+1 in winter and UTC+2 in summer. Any system or communication that stores a time as "Eastern Time" without recording whether DST is active has recorded ambiguous information.
This is why software engineers use IANA timezone identifiers (like America/New_York) rather than names like "Eastern Time." The IANA identifier is associated with a full set of historical and future DST rules, so the software can determine the correct offset for any specific date. The name alone cannot.
Reading an offset
UTC+5:30 — add 5 hours 30 minutes to UTC to get local time. If it is 09:00 UTC, it is 14:30 UTC+5:30.
UTC−5 — subtract 5 hours from UTC. If it is 14:00 UTC, it is 09:00 UTC−5.
UTC+0 — same as UTC. Used in the UK in winter (GMT) and Iceland year-round.
When to use an offset instead of a name
Use an explicit UTC offset any time you are communicating a time to someone in a different timezone and precision matters. Include the offset alongside the timezone name or city for maximum clarity: "09:00 EST (UTC−5, New York)". In API design, log files, and database records, store times in UTC and convert to local time at display only.