Adobe LiveCycle how to calculate time difference

I have a livecycle form with two time fields and a textbox; I am trying to do a calculation between the two time fields.  I have the following code in the formCalc but it will not calculate:

// get end time in milliseconds from the Epoch date
var EndTime = Time2Num(SatTimeOut1.formattedValue, "HH:MM")
// get start time in milliseconds from Epoch date
var StartTime = Time2Num(SatTimeIn1.formattedValue, "HH:MM")
// compute difference in milliseconds
var Diff = EndTime - StartTime
// difference in seconds
Diff = Diff / 1000
// difference in minutes
var Minutes = Diff / 60
// difference in hours
var Hours = Minutes / 60
// Get whole hours only
Hours = Floor(Hours)
// Get minutes only (modulo 60 of minutes)
Minutes = Mod(Minutes, 60)
// format the result
Concat(Hours, ":", Format("99", Minutes))

When I preview the PDF is shows 0:00 in the text field no matter what value is entered in the time fields.

Any help would be greatly appreciated...