Since a nonce must be unique and that time only goes forward, is it safe to use the Unix epoch as nonce ?
My use case is be AES w/ GCM which recommends a nonce length of 12B. Current Unix time can be represented using 4B, so that leaves the upper 8B free for future timestamps (plenty of space).
One possible security implication is the forgery of NTP replies, which would cause the client to use a Unix timestamp in the past (i.e. breaks the stream cipher). Secure channels should prevent this from happening, but a bad operator could always force bad NTP replies.