from_timestamp(long): number (seconds) => string (yyyy-MM-dd HH:mm:ss)
unix_timestamp(String): string (yyyy-MM-dd HH:mm:ss) => number (seconds)
to_timstamp(String):字符串（yyyy-MM-dd HH:mm:ss）=>TimestampType
cast(long as timstamp): number (seconds) => TimestampType
Built-in function source code: org/apache/spark/sql/catalyst/ package