Project 2007 - Copying Start1 value to pjTaskStart using VBA

Dear Gurus,

Apologies first, I'm inexperienced with Project VBA, however from the internet I've managed to get some small code segments working.

I've been struggling with someting that I accept MS Project was not designed to do, basically feed the start and finish dates from my own calculated dates. Reasons are fairly obscure, but I do have such a bizzare need.

Within my macro loop I've got to this point:

Dim i As Long
Dim TempCal As Variant
    For i = 1 To ActiveProject.Tasks.Count Step 1
        If Not ActiveProject.Tasks(i) Is Nothing Then
            If ActiveProject.Tasks(i).GetField(FieldNameToFieldConstant("Start MBJ1")) > 0 Then
                ActiveProject.Tasks(i).SetField pjTaskDuration, "0?"
                TempCal = ActiveProject.Tasks(i).GetField(FieldNameToFieldConstant("Start1"))
                ActiveProject.Tasks(i).SetField pjTaskStart, TempCal
                TempCal = ActiveProject.Tasks(i).GetField(FieldNameToFieldConstant("Finish1"))
                ActiveProject.Tasks(i).SetField pjTaskFinish, TempCal
            End If
        End If
    Next i

Macro appears to compile OK, and runs fine. I stepped through it and it selects the corect lines of the project and correctly sets the pjTaskDuration

With the pjTaskStart and pjTaskFinish, I'm getting nothing but random results. I checked the values of TempCal prior to attempting to SetField, and in both cases the values contained were the correct dates I wished.

Can anybody guide me to my mistake?

Many thanks for your time.