Modules & VBA :: Writing Fields To Table Definition

I am trying to write some fields to an access table definition. It is working for the most part except that I want to write some of the fields as dbText and others as dbDate. For some reason it is writing all of them as dbDate and I don't know why.

Code:
additionalColumns = Array("a","b","c","d","e","f")
'add additonal columns to report
For i = LBound(additionalColumns) To UBound(additionalColumns)
MsgBox additionalColumns(i)
If additionalColumns(i).Value = "a" OR "b" Then
Set columnNames = xlsht.Cells(1, additionalColumns(i))
Set FieldName = tb1.CreateField(additionalColumns(i), dbDate, 10)
tb1.Fields.Append FieldName
Else
Set columnNames = xlsht.Cells(1, additionalColumns(i))
Set FieldName = tb1.CreateField(additionalColumns(i), dbText, 150)
tb1.Fields.Append FieldName
End If
Next i