Forms :: Default Value In Onchange Event

On my form i have a control called WeekDate (this is always the start of the week (monday), below i have a subform (sbfrmWorksheetDetails) contained in a tab control (tabWeek) with 5 pages for each day of the week, i have written a select case statement in the onchange event for tabWeek which selects records based on the day of the week (so = WeekDate for Monday, +1 for Tuesday, +2 for Wednesday etc), this works great, i have also put a bit of code in the same onchange event that makes the default value for a new record for the field WorkDayDate in the subform +1 and +2 etc. this works great for existing records, each new line in the subform puts the right date in, however this seems to cock up when clicking on a new tab that doesn't have a record in..Here's the code.

Private Sub tabWeek_Change()
Select Case tabWeek
Case 0 'Monday
Forms!WeeklyWorksheet!sbfrmWorksheetDetails.Form.R ecordSource = "SELECT * FROM tblWorksheetDetails WHERE (((tblWorksheetDetails.WorkDayDate) = [Forms]![WeeklyWorksheet]![WorksheetDate]))"
Forms!WeeklyWorksheet!sbfrmWorksheetDetails.Form!W orkDayDate.DefaultValue = SQLDate(Me.WorksheetDate)
Case 1 'Tuesday
Same as above but WorksheetDate+ 1 for both select and default value.
Case 2 'Wednesday
Same but +2 etc.
End Select
End Sub

the SQLDate is some code that converts the american US date formatting to UK formatting. i'm not sure if that's the problem, but i'll post that anyway:

Function SQLDate(vDate As Variant) As String
If IsDate(vDate) Then
SQLDate = "#" & Format$(vDate, "dd/mm/yyyy") & "#"
End If
End Function

I can't quite work out what screwing this up upon adding a new record.