Referencing flowEndSysUpTime (21) and flowStartSysUpTime (22) IPFIX Entities in iRule

I am trying to see if I can setup F5 to sent IPFIX data to Cisco Stealthwatch (Lancope). Two required IP Flow Information Export (IPFIX) Entities are flowEndSysUpTime (21) and flowStartSysUpTime (22). I have been following instructions from here to create the IPFIX template iRule.

How do I reference relative timestamps in this iRule? IANA Documentation mentions that this is related to sysUpTime or systemInitTimeMilliseconds on the F5. I am guessing that I need to reference this at the beginning and end of flow, but I am not sure 100% how to do this.

Thanks for your help.