Time Zone Differences and Daylight Saving Time Tips

What is a UTC offset?

Every time zone is defined as an offset from UTC (Coordinated Universal Time), expressed as +/- hours (and sometimes minutes). UTC+9 means 9 hours ahead of UTC.

Formula: Local time = UTC + UTC offset

Example: if Seoul (UTC+9) is 10:00 AM, then London (UTC+0, without DST) is 1:00 AM.

What is Daylight Saving Time (DST)?

In regions that observe DST, clocks are moved forward by 1 hour in spring and back in autumn. During DST, the UTC offset of that region increases by 1.

• US Eastern: Standard UTC-5, DST UTC-4
• Central Europe: Standard UTC+1, DST UTC+2
• Korea, Japan: No DST (always UTC+9)

How to calculate time differences safely

1. Look up the current UTC offset of both locations, including whether DST is active.
2. Calculate the difference between the two offsets.
3. For future scheduling, always verify whether DST will be in effect on that specific date.

Example: Seoul (UTC+9) and New York during DST (UTC-4): difference = 9 − (−4) = 13 hours

New York outside DST (UTC-5): difference = 9 − (−5) = 14 hours

Key takeaways

• All time zones are defined relative to UTC.
• DST regions shift their UTC offset seasonally.
• For international meetings, always base scheduling on the actual offset for that specific date.
• The IANA timezone database is the authoritative source for timezone data.