This class have some diff between flink/blink planner:
- Blink intros some constants (e.g., MICROS_PER_DAY, SECONDS_PER_DAY), inner use, it does not matter.
- Blink intros a function unixDateTimeToString (new)
- Blink changes the behavior of some function
- dateStringToUnixDate: only used in test & codegen now, can be moved into another util class
- timeStringToUnixDate: only used in test & codegen now, can be moved into another util class
- Blink intros USER TimeZone, but now it’s always UTC TimeZone, so it does not matter