Unix epoch time (or Unix time or POSIX time or Unix timestamp) is the number of seconds that have elapsed since January 1, 1970 (midnight UTC/GMT). This can be obtained from the API Gateway time context variables and used in assertions.
The current API Gateway time can be obtained via:
${gateway.time}
The current Unix epoch time can be obtained via:
${gateway.time.seconds}
Other timestamp suffixes can be used to obtain other information; for example:
${gateway.time.millis}
More information on timestamp suffixes can be found via the Policy Manager User Manual and/or Help system, by searching for "Date and Time Variables".