Time zone conversions

Conversion between time zones obeys the relationship

“time in zone A” − “UTC offset for zone A” = “time in zone B” − “UTC offset for zone B”,

in which each side of the equation is equivalent to UTC. (The more familiar term “UTC offset” is used here rather than the term “zone designator” used by the standard.)

The conversion equation can be rearranged to

“time in zone B” = “time in zone A” − “UTC offset for zone A” + “UTC offset for zone B”.

For example, what time is it in Los Angeles (UTC offset= −08) when the New York Stock Exchange opens at 09:30 (−05)?

time in Los Angeles = 09:30 − (−05:00) + (−08:00) = 06:30.

In Delhi (UTC offset= +5:30), the New York Stock Exchange opens at

time in Delhi = 09:30 − (−05:00) + (+5:30) = 20:00.

These calculations become more complicated near a daylight saving boundary (because the UTC offset for zone X is a function of the UTC time).