Why time zones exist
Time zones slice the globe into regions that share the same legal clock time. They are defined by governments, not math, so rules can change whenever a parliament decides. The baseline reference is Coordinated Universal Time (UTC). Every other zone is described as an offset from UTC such as UTC-05:00 or UTC+09:30, which is why India runs UTC+05:30 and Australia keeps half-steps like UTC+10:30 in Lord Howe Island.
To stay accurate you need the IANA zone (e.g., America/Los_Angeles), the current UTC offset, and the
next daylight saving change. That combination removes ambiguity, which is why the TimeyKit
Timezone planner relies on IANA names instead of colloquial abbreviations.
Offsets and abbreviations
IANA zones beat local abbreviations
Abbreviations such as PST, BST, or CST are overloaded. PST can mean Pacific Standard Time (UTC-08:00) or
Philippine Standard Time (UTC+08:00). The abbreviation rarely indicates whether daylight saving is active. By
contrast, America/Los_Angeles is unambiguous because the tz database records the entire timeline of
daylight saving flips for that locality. When you feed that name into a scheduler, the library can evaluate the
correct offset for any date, including future transitions.
Example: Seattle vs Singapore
Suppose a Seattle engineer (America/Los_Angeles) wants to meet a Singapore teammate (Asia/Singapore). On February 22, 2026, Los Angeles sits at UTC-08:00 while Singapore is UTC+08:00, a 16-hour gap. If the Seattle person proposes 08:00 their time, it lands at midnight in Singapore. Once daylight saving begins on March 8, Los Angeles moves to UTC-07:00, narrowing the gap to 15 hours. Storing IANA names keeps this adjustment automatic; plain text invites with "PST" do not.
Daylight saving ripple effects
Roughly 70 regions still observe daylight saving time (DST). They jump forward in spring and fall back in autumn, but no two regions shift on the same weekend. The United States moves ahead on the second Sunday in March, the United Kingdom waits until the last Sunday in March, and Queensland in Australia never changes. During the weeks between those transitions your overlap window shifts, often without warning to your calendar.
The safest routine is to log the exact date you care about, pick the remote teammate’s IANA zone, and read the resulting overlap grid. The Timezone tool highlights shared office hours and shows how DST alters them. If you need a live visual for a broadcast day, pin both cities to the World Clock wallboard and leave it open on a side monitor.
Example: US vs UK DST gap
Imagine a daily stand-up between New York (America/New_York) and London (Europe/London). In mid-March the U.S. has sprung forward but the UK has not. For two weeks the usual 5-hour difference shrinks to 4 hours, so a 09:00 New York stand-up shifts from 14:00 UK to 13:00 UK. If you schedule without checking, your UK crew wonders why the invite suddenly begins during lunch.
Building reliable workflows
Store base times in UTC
Whether you maintain event data in a database or a spreadsheet, persist timestamps in UTC and convert only when
displaying. UTC ignores daylight saving, so math stays predictable. When you need a human-friendly slot, convert
with Intl.DateTimeFormat or the TimeyKit Epoch converter.
Tests and runbooks
Document which zones matter to your team and capture ambiguous weeks. Keep a “DST gotchas” page describing what to watch each March and October, plus a saved Timezone link so responders can see overlaps in a click.
Try the tool
Ready to model your scenario? Open the Timezone planner for side-by-side schedules or load the World Clock wallboard for an always-on display. Both tools respect IANA zones, daylight saving changes, and oddball offsets like UTC+05:45.
FAQ
- Why do some zones have half-hour offsets?
- Local governments set their own legal time. India, Nepal, and parts of Australia chose half-hour or quarter-hour offsets to stay centered on the sun without joining a neighbor’s standard time.
- Is GMT the same as UTC?
- Greenwich Mean Time historically referred to solar time at the Royal Observatory. Modern civil timekeeping uses UTC, so most planners treat GMT as UTC except during British Summer Time when the UK becomes UTC+01:00.
- Do calendar apps handle DST automatically?
- Yes, if the event is stored with a timezone that includes IANA rules. Problems arise when invites are created with floating times (no zone) or plain text instructions such as “call me at 9am PST”.
- How often do time zone laws change?
- Every year several countries adjust DST windows or entire offsets. Always refresh your data set from the tz database and confirm against a trusted planner when high-stakes events are on the line.