Coverity 1508534 & 1508540: misuses of time_t

Coverity 1508534 & 1508540:  misuses of time_t

Avoid problems when the lower 32 bits of time_t roll over by delaying
the cast to integer until after the time delta has been computed.

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19004)

(cherry picked from commit a6cadcbdc3b4f3fbd0fd228e41177f0661b68264)