ConstructTimeZone constructs timezone by name first. When the timezone name is set, the daylight saving problem must be considered. Otherwise the timezone offset in seconds east of UTC is used to constructed the timezone.
GetSystemTZ gets the value of systemTZ, an error is returned if systemTZ is not properly set.
InferSystemTZ reads system timezone from `TZ`, the path of the soft link of `/etc/localtime`. If both of them are failed, system timezone will be set to `UTC`. It is exported because we need to use it during bootstrap stage. And it should be only used at that stage.
LoadLocation loads time.Location by IANA timezone time.
SetSystemTZ sets systemTZ by the value loaded from mysql.tidb.
SystemLocation returns time.SystemLocation's IANA timezone location. It is TiDB's global timezone location.
WithinDayTimePeriod tests whether `now` is between `start` and `end`.
Zone returns the current timezone name and timezone offset in seconds. In compatible with MySQL, we change `SystemLocation` to `System`.