How to autonumber date values in a control.

Private Sub dtMonths_GotFocus()
    
    ' to avoid errors from data validation, dim the date
    ' variable as type Variant
    Dim dtDateValue As Variant 
 
    Dim dtCurrentDate As Date
    Dim dtPriorDate As Date
    
    ' what's in the field right now?  dtMonths is the name 
    ' of the date field
    dtDateValue = Me!dtMonths.Value

    ' if a date isn't in the current field, find out what's
    ' in the same field in the prior record
    If Not IsDate(dtDateValue) Then
        dtPriorDate = CheckPriorDateValue()

        ' calculate what the current field should be
        ' in this example, the month value is incremented 
        ' by one
        dtCurrentDate = DateAdd("m", 1, dtPriorDate)
        
        ' assign the new value to the current field
        Me!dtMonths.Value = dtCurrentDate
    End If
    
End Sub

CheckPriorDateValue()

Private Function CheckPriorDateValue() As Date
        
    Dim dtPriorDateValue As Date
    Dim strSQL As String
    
    On Error GoTo ERROR_HANDLER
    
    strSQL = "SELECT * FROM YourTable(s) WHERE criteria" & _
    " = requirments"
    
    Dim cnCurrent As ADODB.Connection
    Dim rsDates As ADODB.Recordset
    
    Set cnCurrent = CurrentProject.Connection
    Set rsDates = New ADODB.Recordset
    
    ' open the recordset
    rsDates.Open strSQL, cnCurrent, adOpenDynamic, _
       adLockOptomistic

    ' move to the last record and get the value
    rsDates.MoveLast
    dtPriorDateValue = rsDates!dtMonths
    
    ' close the recordset and connection
    rsDates.Close
    cnCurrent.Close
    Set rsDates = Nothing
    Set cnCurrent = Nothing
    
    ' return the prior date value
    CheckPriorDateValue = dtPriorDateValue
    Exit Function
    
ERROR_HANDLER:
    ' an error occurs when a recordset can't be created. 
    ' e.g.  no data
    ' this means that the focused "dtMonth" MUST be the 
    ' first date value in the querried table.  therefore,
    ' the first month to appear in the field should be
    ' the current date.
    
    ' assign today to dtPriorDateValue
    dtPriorDateValue = Format(Date, "mmm yy")
    ' turn it into a prior date and return
    CheckPriorDateValue = DateAdd("m", -1, _
       dtPriorDateValue)  'e.g., last month
    Exit Function
    
End Function