When should I use time synchronization option?

Without that option, if someone captured initial handshake packet from the valid client, then it could repeat it and receive a handshake packet as an answer from server. This can be used to detect if some user’s identity is known to server and that it is exactly GoVPN server. Timesync option forces adding timestamps to each handshake packet allowing only specified time window of packet validness. If you have got more or less synchronized clocks between your clients and server, then you can always use this option.