SST support:

The Session-Expires header conveys the session interval for a SIP call. It is placed in an INVITE request and is allowed in any 2xx class response to an INVITE. Its presence indicates that the UAC wishes to use the session timer for this call. Unlike the SIP-Expires header, it can only contain a delta-time, which is the current time, plus the session interval from the response.

For example, if a UAS generates a 200 OK response to a re-INVITE that contained a Session-Expires header with a value of 1800 seconds (30 minutes), the UAS computes the session expiration as 30 minutes after the time when the 200 OK response was sent. For each proxy, the session expiration is 30 minutes after the time when the 2xx was received or sent. For the UAC, the expiration time is 30 minutes after the receipt of the final response.