Question 1

What is the difference between UTC and GMT?

Accepted Answer

UTC (Coordinated Universal Time) and GMT (Greenwich Mean Time) are nearly identical for everyday use: both sit at the zero meridian with no UTC offset. The technical difference is that GMT is a timezone based on mean solar time at the Royal Observatory in Greenwich, while UTC is an atomic time standard. Most programming contexts use UTC.

Question 2

How does daylight saving time affect Unix timestamps?

Accepted Answer

Unix timestamps are not affected by daylight saving time because they are always measured in UTC seconds. DST only affects the display of a timestamp when it is converted to a local timezone. This converter applies the correct DST offset for the selected timezone and date automatically.

Question 3

What is an IANA timezone identifier?

Accepted Answer

An IANA timezone identifier (also called an Olson timezone name) is a string like 'America/New_York' or 'Asia/Tokyo' that uniquely identifies a timezone region and its full history of UTC offsets and DST rules. They are maintained in the IANA Time Zone Database and supported by all major operating systems and programming languages.

Question 4

Should I store UTC or local time in my database?

Accepted Answer

Always store UTC (or Unix timestamps) in your database. Store the user's timezone preference separately and convert to local time only at display time. This avoids ambiguity during DST transitions and makes cross-timezone comparisons reliable.

时区转换器

IANA 时区数据库

夏令时(DST)

UTC 偏移详解