A Simple Guide to Time Zones and Calculating Time Differences
Scheduling a call across countries is easy to get wrong, especially when daylight saving time is involved. A little understanding of how time zones work removes the guesswork.
UTC is the reference
Coordinated Universal Time (UTC) is the world's time standard. Every local time is expressed as an offset from UTC. For example, UTC+5 is five hours ahead of UTC, and UTC−5 is five hours behind. Pakistan Standard Time is UTC+5, India Standard Time is UTC+5:30, and Japan is UTC+9.
Not all offsets are whole hours
India (+5:30), Nepal (+5:45) and parts of Australia (+9:30 in some regions) use half-hour or 45-minute offsets, so never assume whole hours.
The method for converting
- Find the offset of each place for the specific date.
- Convert the source time to UTC by subtracting its offset.
- Add the destination offset to get the local time there.
- Check whether the result moves into the previous or next day.
Example: 15:00 in UTC+5 → 15 − 5 = 10:00 UTC. In UTC−4 that is 10 − 4 = 06:00 the same day. In UTC+9 it is 10 + 9 = 19:00.
Daylight saving time (DST)
Many countries in North America and Europe move their clocks forward in spring and back in autumn, while many others, including most of Asia, do not. The change dates differ between regions, and for a few weeks the usual time difference between two cities may shift by an hour. Always check the specific date of the meeting, not just the usual difference.
Time zone names
Abbreviations can be ambiguous. "CST" can mean Central Standard Time in North America, China Standard Time or Cuba Standard Time. Use city names or UTC offsets in invitations: "10:00 Karachi (UTC+5)".
Tips for scheduling
- Use a calendar that adds time zones automatically.
- Propose times in the recipient's local time as well as yours.
- For recurring meetings across DST changes, confirm the time after the clocks change.
- Find the overlapping working hours. Between Asia and the Americas, early morning for one side is often evening for the other.
The International Date Line
Crossing it changes the calendar date. A flight departing Auckland on Tuesday can land in Los Angeles on Monday. Flight tickets show local times, so calculate duration by converting both to UTC.
Date arithmetic
For differences between dates, remember that months have different lengths and leap years add February 29. A calculator that handles days, months and years avoids manual mistakes.
State times with an offset and a date, and misunderstandings largely disappear.
