According to documentation you should be able to set a variable with the seconds timestamp.
This is needed to set a JWT validity with a time value + 1 day (offset).
This does not work as the variable is always returned as UTC formatted.
The format tab is used to choose a format from the drop-down list. The assertion uses this format to interpret the value in the Expression field. The default is "<auto>", which means the assertion tries to determine the format automatically.
To get the variable time value in seconds (Epoch Time ) you can use the following syntax.
${timevariablename.seconds}
https://techdocs.broadcom.com/us/en/ca-enterprise-software/layer7-api-management/api-gateway/10-1/reference/context-variables/date-and-time-variables.html