I am using below but I need the output to be in decimal time. This is for travel computations and our travel will often travel over the 24:00 time mark.

// Time Values
   var cStartTime = this.getField("DT OT L1").value;
   var cEndTime = this.getField("AT OT L1").value;

   // Only process if field contains a value
    if ((cStartTime != "") && (cEndTime != "")) {
    // Convert to Hours Decimal value
    var nStartTime = 0,
        nEndTime = 0;


    var aStartTime = cStartTime.split(":");
    nStartTime = Number(aStartTime[0]) + Number(aStartTime[1]) / 60;
    var aEndTime = cEndTime.split(":");
    nEndTime = Number(aEndTime[0]) + Number(aEndTime[1]) / 60;



   // Find Difference
    var nTimeDiff = nEndTime - nStartTime;

   // Test for Midnight Crossover
    if (nTimeDiff < 0) { // Shift 24 hours
        nTimeDiff += 24;
    }


   // If used in a calculation, may need to be changed to "event.value ="
    var nHours = Math.floor(nTimeDiff);
    var nMinutes = Math.floor((nTimeDiff - nHours) * 60 + 0.5);
    event.value = util.printf("%02d:%02d", nHours, nMinutes);
    } else
    event.value = "";