[bbojan]

You mean you must record the timezone? Because the TZ offset can change throughout the year (e.g. due to daylight saving time).

[hughw]

Usually the TZ offset is enough. The use case is a guy reading notes and logs from multiple sources. All that guy needs is to see the local time at the time and place of the event. So that, for example, he can match up the time stamp on some computer recorded log with the time some human operator recorded, in local time, on a paper record.