Time Zone Conversion: UTC, DST, IANA Timezones & Meeting Overlap Explained

UTC offset and timezone sound like the same thing — they are not. A UTC offset is a number. A timezone is a rulebook. Understanding the difference is what separates a conversion that works from one that is wrong by exactly one hour twice a year.

UTC: the fixed reference

UTC (Coordinated Universal Time) is the global reference clock. It has no daylight saving offset and never changes — every other timezone is defined as a positive or negative offset from it. When a developer stores a timestamp as UTC, they are storing a single unambiguous moment in time that can be correctly rendered in any local timezone.

You will also see GMT (Greenwich Mean Time) used interchangeably with UTC. For practical purposes they are the same — GMT is the historical predecessor, UTC is the modern standard. The difference (a few fractions of a second due to leap seconds) is irrelevant for everyday scheduling.

UTC offset vs IANA timezone name

A UTC offset (e.g. UTC+5:30) is a fixed number describing how far a clock is from UTC at a specific instant. The problem: it changes. London is UTC+0 in winter and UTC+1 in summer. If you store a meeting as "UTC+1, 14:00" and then open it in December, you are off by an hour.

An IANA timezone name (e.g. Europe/London) is a named region from the IANA Time Zone Database — also called the Olson or tz database. The name encodes the complete history of every offset and DST rule that has ever applied to that region. When you convert using an IANA name, the correct offset for the exact date is applied automatically, even across DST boundaries.

Always store and convert with IANA timezone names, not UTC offsets. UTC offsets are only appropriate for display.

How daylight saving time works

Daylight saving time shifts clocks forward by one hour in spring ("spring forward") and back in autumn ("fall back"). About 70 countries observe DST, but on different dates:

• United States: second Sunday in March → first Sunday in November
• European Union: last Sunday in March → last Sunday in October
• Australia: first Sunday in October → first Sunday in April (southern hemisphere — opposite cycle)

Countries near the equator (India, UAE, Singapore, Japan, China) do not observe DST. Russia abolished DST in 2014 and stays on permanent standard time.

The US–EU gap creates a confusing two-week window each spring and autumn where the UTC offset difference between, say, New York and London is one hour different from normal. During these windows, a recurring meeting that normally falls at a workable overlap time can shift into off-hours for one side.

Why some timezones use 30 or 45-minute offsets

Most timezones use whole-hour offsets, but some use fractional offsets for historical or political reasons:

• India (IST): UTC+5:30 — chosen in 1947 to split the difference between its eastern and western extents
• Nepal (NPT): UTC+5:45 — set 15 minutes ahead of India to distinguish itself
• Iran (IRST): UTC+3:30 in winter, UTC+4:30 with DST
• Australia Central (ACST): UTC+9:30 — a legacy of 19th-century telegraph time
• Chatham Islands (NZCHT): UTC+12:45 — the most unusual offset in daily use

Finding a meeting time across timezones

The meeting planner in this tool works by checking each UTC hour against your configured work-hours window for every participant timezone. An hour qualifies as an overlap window only when every participant falls within their local work hours at that UTC time.

Practical tips for remote scheduling:

• New York + London: the best overlap is 14:00–17:00 UTC (NY morning, London afternoon)
• London + Singapore: limited — try 08:00–09:00 UTC (London morning, Singapore evening)
• San Francisco + Dubai: nearly impossible for a standard 9–5 window — one side needs to flex
• For async-first teams, record a Loom instead of forcing off-hours calls