Question

how is the standard time of a place determined?

Answer 1

First of all, if someone told you it can be calculated from the longitude of the place, they lied (perhaps unintentionally). That method has been obsolete since countries started forming time zones in the 19th century.

You need to use a time zone database. I like Current Local Times Around the World. You could also use a time zone map, but many don’t show what places use daylight saving time, and those that do don’t show when it is in effect.