How to retrieve Unix Timestamp

Invoke NETWORK::_GET_POSIX_TIME / 0x9A73240B49945C76. Don't confuse this with TIME::GET_POSIX_TIME, which doesn't return Posix/Unix time.

int NETWORK::_GET_POSIX_TIME()

In FiveReborn

As of the time of writing, FiveReborn's natives.lua doesn't add functionality for this native. You'll probably have to invoke it like below.

Citizen.InvokeNative(0x9A73240B49945C76, Citizen.ResultAsInteger())