Is there a better way?

Setup, Configuration and Use Scripts & Rules
3

This is not tested and may contain some errors, but this is basically the easiest way since openHAB2.2

rule "Update User1 Last-Seen Time"
when 
    // this will get easier in future when the group trigger is accepted 
    // and added to the distribution, then only the group has to be listed
    Item Presence_Mobile_User1 changed or 
    Item Presence_Mobile_User2 changed or  
    Item Presence_Mobile_User3 changed
then
    var time = new DateTimeType() 
    postUpdate(  triggeringItem.name.toString , time.toString)
end

there are some things which maybe will be of interrest in following threads, for your rules. I tried to add as many explanations as a could.

or here