This is probably a good time to plug (or a release timed to coincide with) the NTPSec project [1], which is approaching actual launch goals [2].

Time is incredibly important for security. As a wise man once said, you can't hide secrets from the future [3]. You can only protect them with the best crypto you currently have, and expire old secrets as they are no longer valid. Kerberos, most OTP protocols, and SSL are all predicated on the two sides having a consistent view of time. SSL Certs have an expiry, for reasons that include that we expect their crypto to fail. Kerberos uses it among other things to prevent and limit scope of replay attacks, as does the TOTP protocol.

[1] http://www.ntpsec.org/ [2] https://plus.google.com/+EricRaymond/posts/WHQ4YMTy14U [3] https://youtu.be/NK5WfcmeaUg