Excel formulas: increments months or years or days

Incrementing Months in Excel

If cell A1 = "06/01/2013" and A2 contained formula: 

=DATE(YEAR(A1),MONTH(A1)+18,DAY(A1)) 

...A2 would then display "06/19/2013"

Also...

Unsure if a "date" in Excel is truly a date and not some screwed up string value?

Format it as "mm/dd/yyyy".  If it doesn't conform, its still a date.  

To make it manually conform, try typing it in manually.  Otherwise, resort to VBA code:

Sub EstablishDates()
    On Error GoTo err_EstablishDates

    Dim oWS As Excel.Worksheet
    Set oWS = Application.Worksheets("Sheet2")
    
    Dim iRow As Integer
    
    For iRow = 2 To 20
    
        If oWS.Range("I" & iRow).Value = "" Then Exit For
    
        oWS.Range("I" & iRow).Value = CDate(oWS.Range("H" & iRow).Value)
            
    Next

    Set oWS = Nothing

exit_EstablishDates:
    Exit Sub
err_EstablishDates:
    MsgBox Err.Number & " " & Err.Description
    GoTo exit_EstablishDates

End Sub