There are commands that define the lifetimes of the ISAKMP and IPSec Security Associations (SAs).

E.g.:

crypto isakmp policy 1
encr 3des
authentication pre-share
group 2
lifetime 3600

crypto ipsec security-association lifetime seconds 3599

... and you can determine lifetime remaining for these SAs with the following commands:

sh crypto session detail

sh crypto isakmp sa detail

sh crypto ipsec sa

The delta between the configured lifetime(s), and the lifetime remaining will tell you how much time has passed since the last re-keying, but that is as close as you are likely to get in determining when the tunnel first came up.

You might use other means such as syslog to tell you when a Tunnel transitions to Up or Down state(s).

Best Regards,

Mike