When I hit a situation like this, I fall into JavaScript, with something like
DateTime Presence_RobertPhone_Timestamp "Timestamp [%1$tm/%1$td %1$tH:%1$tM]" <clock> (Presence_RobertPhone) {mqtt="<[ext:owntracks/robert/iphone:state:JS(owntime.js)]"}
transform/owntime.js (untested):
(function(json){
var otdata = JSON.parse(json);
return new Date(otdata.tst * 1000).toISOString();
})(input)