Using the Unix epoch as nonce
Using a Unix Timestamp as the sole source for the nonce would make me nervous.

In addition to forged NTP replies (and legitimate operators deliberately resetting the clock for some reason, and I'm not sure whether leap seconds would pose a risk), you also would need to worry about "what if I send two messages within the same second"

On the other hand, you have 12 bytes to play with; if you use four of the bytes to hold the Unix timestamp, and use another four as a counter (which you increment every time you send a message), and the other four as something else, that'd be a system which would be more resilient against nonce repeats.