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.

ParseTimeZone parses the time zone string, returns the location and whether the time zone is valid.

SetSystemTZ sets systemTZ by the value loaded from mysql.tidb.

Sleep is a function like time.Sleep(), the difference is it will return early when ctx finished.

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.

ErrUnknownTimeZone indicates timezone is unknown.