The client interface to the time zone server.

This class performs two basic functions:

1. Converts between UTC time and local time. 2. Sets the current local time zone.

NOTE: The presence of a time zone server will alter the behaviour of the time zone related function calls User::SetUTCOffset() and User::SetUTCTimeAndOffset(). The time zone server is shutdown when the last session (RTz) is closed. Therefore, to maintain consistent time related behaviour, licensees may want to keep a system level time zone server session open at all times.

Exceptional cases occur when a user requests conversion for a non-existent local time or a double local time.

Non-existent local times occur when the local time changes from winter to summer for DST.

For example, 01:59 local time is non-existent on the day of a change to BST in Europe/London since the time changes directly from 12:59:59 to 2:00.

A double local time occurs when the local time changes from summer to winter.

For example, if the time changes at 02:00 AM BST to 01:00 AM GMT then local times between 01:00 and 01:59 occur twice.

The conversion applies the DST offset if the local time value is double and applies the standard UTC offset if the local time does not exists.

This decision makes the conversion process asymmetrical around the discontinuity in the local time when there is a DST change.

An example conversion from a double local time to UTC and from UTC to a double local time is:

01:59 AM BST => 00:59 AM UTC 01:59 AM UTC => 01:59 AM GMT

An example conversion from a non-existent local time to UTC and from UTC to local time is:

01:59 AM GMT => 01:59 AM UTC 01:59 AM UTC => 02:59 AM BST

9.1