UK Date Format ComboBox

I'm having trouble getting a combo box to show UK format dates. It's meant to check the last date on the spreadsheet, find so many weeks before and after and fill the drop-down. It finds the weeks fine, I've even had it put them into a column in the spreadsheet and they show fine there, but the drop-down always comes up with US format dates even if I read it from the test column in the spreadsheet. It then writes the US format date into the spreadsheet, which causes mayhem next time I run it, as I'm forcing it to read UK style dates in an attempt to make it work. It sometimes does decide to work fine, I'm never sure why, but it changes back to being wrong again when I close and restart it. I've checked my regional settings, and they're right.

Dim i As Integer
Dim ThisWeek As Date
Dim rCell As Range
Dim UsedWeek As Date
FirstTime = False
For i = 0 To 20
Redim Week(i) As Date
Next i
ThisRow = Cells(Cells.Rows.Count, "A").End(xlUp).Row+1
If ThisRow = 9 Then............