Convert between timestamp in milliseconds and Date, and vice verse.

Timestamp: Every federation transaction/message is timestamped with the respective servers date and time.
The timestamp is measured as the total number of milliseconds since 1 January 1970 also known as Unix time / POSIX time or epoch time. Convert back and forth between milliseconds and datetime.
Datetime (UTC format)
Milliseconds (Unix time / POSIX time or epoch time)