Is it safe to use a timestamp to build a nonce?
This isn't a very good idea - your chances of collision are not that low (depending on the number of clients you have). At best, your chances are $2^{-32}$ for 2 active clients. You would be better of just using a secure random 96 bit nonce as recommended by the NIST guidelines for AES-GCM (note though that you must not send more than $2^{32}$ messages with the same key when using this construction so that the chance of IV colliding doesn't exceed $2^{-32}$)