Duration Field value in Process Target is Invalid

Hello experts,

I have a problem with this "Duration in Record":
I have created Process Target which is filled correctly in Incident:
SLO.JPGSLO_error.JPG

I have used script:

function ConvertDate(end, start){

var End = new Date(end)
var Start = new Date(start)
var Duration = End-Start
var Duration = Duration/1000/60

ConDuration= new XMLDate(Duration)
return ConDuration.getSCDateTimeString()
}

To store "time" inside of that field which for minutes look's like this '00:11:00' I have tried to push data like this '1 00:01:00' and it didn't accept it.
Does anyone know what could be the problem?
Best regards,
Dino