Timesheet calculations - Where am I going wrong?

I've been batting this one back and forth for weeks now and every time I think I've finally nailed it something breaks it.

Entering different values can cause terrible trouble for the calculations.  It's all based on a cheat's way of calculating time by using left and right to capture the values for hours and minutes and then using them in a calculation before glueing them together using concatenation.

It seems to handle minus figures up to a point and when I halve the standard working hours things can go very wrong.

Here's a full list of the scripts used.  I'd be grateful if someone could help identify what I need to do to catch and fix these anomalies.

Calculating a Working Day

if      (HasValue(form1.flow.page.parent.part2.part2body.timesheet.Mon.endPM) & HasValue(form1.flow.page.parent.part2.part2body.timesheet.Mon.startAM))

then   

//      Compute time difference in minutes

var     TimeDiff = ((Time2Num(form1.flow.page.parent.part2.part2body.timesheet.Mon.endPM.formattedValue, "H:MM") - Time2Num(form1.flow.page.parent.part2.part2body.timesheet.Mon.startAM.formattedValue, "H:MM")) / (1000 * 60)

        - (Time2Num(form1.flow.page.parent.part2.part2body.timesheet.Mon.startPM.formattedValue, "H:MM") - Time2Num(form1.flow.page.parent.part2.part2body.timesheet.Mon.endAM.formattedValue, "H:MM")) / (1000 * 60))

//      truncate to hours

var     HourDiff = Floor(TimeDiff/60)

//      get minutes less than 60

var     MinDiff = Mod(TimeDiff,60)

//      build formatted string

var     netTime = concat(Format("Z9", HourDiff), ":" , Format("99",MinDiff))

//var     Final = concat((HourDiff), ":" , (MinDiff))

elseif      (HasValue(form1.flow.page.parent.part2.part2body.timesheet.Mon.startAM) & HasValue(form1.flow.page.parent.part2.part2body.timesheet.Mon.endAM) & (form1.flow.page.parent.part2.part2body.timesheet.Mon.endPM.rawValue==null))

then   

//      Compute time difference in minutes

var     TimeDiff = ((Time2Num(form1.flow.page.parent.part2.part2body.timesheet.Mon.endAM.formattedValue, "H:MM") - Time2Num(form1.flow.page.parent.part2.part2body.timesheet.Mon.startAM.formattedValue, "H:MM")) / (1000 * 60))

//        - (Time2Num(form1.flow.page.parent.part2.part2body.timesheet.Mon.startPM.formattedValue, "H:MM") - Time2Num(form1.flow.page.parent.part2.part2body.timesheet.Mon.endAM.formattedValue, "H:MM")) / (1000 * 60))

//      truncate to hours

var     HourDiff = Floor(TimeDiff/60)

//      get minutes less than 60

var     MinDiff = Mod(TimeDiff,60)

//      build formatted string

var     netTime = concat(Format("Z9", HourDiff), ":" , Format("99",MinDiff))

//var     Final = concat((HourDiff), ":" , (MinDiff))

else

//   if any values missing null the output

null

endif

Calculating Weekly Hours

var MonMin = Right(form1.flow.page.parent.part2.part2body.timesheet.Mon.monTotal, 2)

var TueMin = Right(form1.flow.page.parent.part2.part2body.timesheet.Tues.tueTotal, 2)

var wedMin = Right(form1.flow.page.parent.part2.part2body.timesheet.wed.wedTotal, 2)

var thurMin = Right(form1.flow.page.parent.part2.part2body.timesheet.thur.thurTotal,2)

var friMin = Right(form1.flow.page.parent.part2.part2body.timesheet.fri.friTotal,2)

var TotalMin = sum(MonMin, TueMin, wedMin, thurMin, friMin)

var MonHour = Left(form1.flow.page.parent.part2.part2body.timesheet.Mon.monTotal,2)

var TueHour = Left(form1.flow.page.parent.part2.part2body.timesheet.Tues.tueTotal,2)

var WedHour = Left(form1.flow.page.parent.part2.part2body.timesheet.wed.wedTotal,2)

var ThuHour = Left(form1.flow.page.parent.part2.part2body.timesheet.thur.thurTotal,2)

var FriHour = Left(form1.flow.page.parent.part2.part2body.timesheet.fri.friTotal,2)

var TotalHour = sum(MonHour, TueHour, WedHour, ThuHour, FriHour)

var Minutes = Mod(TotalMin,60)

var FinTot  = floor((TotalMin / 60 )) + (TotalHour)

var MinRem

if (Minutes < 10) then

    MinRem = concat("0", Minutes)

else

    MinRem = Minutes

endif

$.rawValue = concat(FinTot, ":", MinRem)

Calculating Hours Gained or Lost over the Week

var item1 = ((left(form1.flow.page.parent.part2.part2body.timesheet.total.weekTotal, 2)) + (left(form1.flow.page.parent.part2.part2body.authAbs,2)))

var item2 = ((right(form1.flow.page.parent.part2.part2body.timesheet.total.weekTotal,2)) + (right(form1.flow.page.parent.part2.part2body.authAbs,2)))

var itemA = left(form1.flow.page.parent.part2.part2body.std.stdHours,2)

var itemB = right(form1.flow.page.parent.part2.part2body.std.stdHours,2)

var hourDiff = (item1 - itemA)

var minDiff  = (item2 - itemB)

var final

if     ((hourDiff < 0) and (item2 > 0))

then    hourDiff = ((hourDiff)-- 1)

        minDiff = (60 - item2)

        final = concat(hourDiff, ":", minDiff)

elseif  ((hourDiff > 0) and (item2 > 0))

then    final = concat(hourDiff, ":", minDiff)

elseif  ((hourDiff > 0) and (item2 == 0))

then    final=concat(hourDiff,":","0",minDiff)

endif

$.rawValue = final

Calculating the Total Number of Hours worked over the four week period

var W1Min = Right(form1.flow.page.parent.part2.part2body.timesheet.total.weekTotal, 2)

var W2Min = Right(form1.flow.page.parent.part3.part3body.timesheet2.total.weekTotal, 2)

var W3Min = Right(form1.flow.page.parent.part4.part4body.timesheet3.total.weekTotal, 2)

var W4Min = Right(form1.flow.page.parent.p5.part5body.timesheet3.total.weekTotal,2)

var TotalWMin = sum(W1Min, W2Min, W3Min, W4Min)

var W1Hour = Left(form1.flow.page.parent.part2.part2body.timesheet.total.weekTotal,2)

var W2Hour = Left(form1.flow.page.parent.part3.part3body.timesheet2.total.weekTotal,2)

var W3Hour = Left(form1.flow.page.parent.part4.part4body.timesheet3.total.weekTotal,2)

var W4Hour = Left(form1.flow.page.parent.p5.part5body.timesheet3.total.weekTotal,2)

var TotalWHour = sum(W1Hour, W2Hour, W3Hour, W4Hour)

var Minutes = Mod(TotalWMin,60)

var FinTot  = floor((TotalWMin / 60 )) + (TotalWHour)

var MinRem

if (Minutes < 10) then

    MinRem = concat("0", Minutes)

else

    MinRem = Minutes

endif

$.rawValue = concat(FinTot, ":", MinRem)

Calculating the Total Number of Conditioned Hours

var c1Min = Right(form1.flow.page.parent.part2.part2body.std.stdHours, 2)

var c2Min = Right(form1.flow.page.parent.part3.part3body.std2.stdHours, 2)

var c3Min = Right(form1.flow.page.parent.part4.part4body.std2.stdHours, 2)

var c4Min = Right(form1.flow.page.parent.p5.part5body.std2.stdHours,2)

var TotalCMin = sum(c1Min, c2Min, c3Min, c4Min)

var c1Hour = Left(form1.flow.page.parent.part2.part2body.std.stdHours,2)

var c2Hour = Left(form1.flow.page.parent.part3.part3body.std2.stdHours,2)

var c3Hour = Left(form1.flow.page.parent.part4.part4body.std2.stdHours,2)

var c4Hour = Left(form1.flow.page.parent.p5.part5body.std2.stdHours,2)

var TotalCHour = sum(c1Hour, c2Hour, c3Hour, c4Hour)

var Minutes = Mod(TotalCMin,60)

var FinTot  = floor((TotalCMin / 60 )) + (TotalCHour)

var MinRem

if (Minutes < 10) then

    MinRem = concat("0", Minutes)

else

    MinRem = Minutes

endif

$.rawValue = concat(FinTot, ":", MinRem)

Calculating the Difference Between Total Hours and Conditioned Hours

//Establish variables for calculations

var minGain     = Right(form1.flow.page.parent.part2.part2body.hoursGain,2) // Gets the last two digits of the Hours Gained field to establish the number of minutes

var weekMin     = Right(form1.flow.page.parent.p6.part6.totalHours, 2)      // Gets the last two digits of the Total Hours field to establish the number of minutes

var totalMin    = sum(minGain,weekMin)                                      // Add the minutes together

var hrGain      = Left(form1.flow.page.parent.part2.part2body.hoursGain,2) // Gets the first two digits of the Hours Gained field to establish the number of hours

var weekHr      = Left(form1.flow.page.parent.p6.part6.totalHours,2)        // Gets the first two digits of the Total Hours field to establish the number of hours

var totalHours  = sum(hrGain,weekHr)                                        // Adds the number of hours together

var Minutes     = Mod(totalMin,60)                                         

var finalTotal  = floor((totalMin / 60 )) + (totalHours)

varminRem

if      (Minutes < 10 )

then    minRem = concat("0",Minutes)

else    minRem = Minutes

endif

//$.rawValue = concat(finalTotal,":",minRem)

var    totHrs          =   Left(form1.flow.page.parent.p6.part6.totalHours,3)

var     totMins         =   Right(form1.flow.page.parent.p6.part6.totalHours,2)

var     condHrs         =   Left(form1.flow.page.parent.p6.part6.textField,3)

var     condMins        =   Right(form1.flow.page.parent.p6.part6.textField,2)

var     gainLossHrs     =   (totHrs - condHrs)

var     gainLossMins    =   (totMins - condMins)

var     gainLossRem

if      (gainLossMins < 10)

then    gainLossRem= concat("0",gainLossMins)

else    gainLossRem = gainLossMins

endif

$.rawValue  = concat(gainLossHrs,":",gainLossRem)