ConstructTimeZone constructs timezone by name first.

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`.

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.

WithinDayTimePeriod tests whether `now` is between `start` and `end`.

Zone returns the current timezone name and timezone offset in seconds.