Barb,
             Perhaps something along these lines. For this example, I set
SummaryMin as the default date.

Sub InForm()

Dim SummaryMin As Date, CPI_SPI_PITD_LastMo As Date, GetDate as Date

SummaryMin = "01/01/2006"
CPI_SPI_PITD_LastMo = "31/01/2006"

Do
getDate = DateValue(Application.InputBox("Enter Date between " & SummaryMin
& " and " & CPI_SPI_PITD_LastMo, "Title date ", SummaryMin, Type:=2))
If getDate = "False" Then Exit Sub  ' Cancel
If Not Application.And(getDate >= SummaryMin, getDate <=
CPI_SPI_PITD_LastMo) Then
  MsgBox "Invalid date. Please re-enter"
End If
Loop Until Application.And(getDate >= SummaryMin, getDate <=
CPI_SPI_PITD_LastMo)
End Sub

One way:

(Your post is a bit confusing - do you want to "enter a date between the
following two dates", or "select one of the two"? I assumed the former:

   Const SummaryMin = #1/1/2006#
   Const CPI_SPI_PITD_LastMo = #12/31/2006#
   Dim vResult As Variant
   Dim dtFirst As Date
   Dim dtLast As Date
   dtFirst = SummaryMin
   dtLast = CPI_SPI_PITD_LastMo
   Do
       vResult = Application.InputBox( _
                   Prompt:="Enter a date between " & dtFirst & _
                           " and " & dtLast & ":", _
                   Title:="Enter Date", _
                   Default:=dtFirst, _
                   Type:=1)
       If vResult = False Then Exit Sub 'user cancelled
       vResult = vResult - ActiveWorkbook.Date1904 * 1462
   Loop Until (vResult >= dtFirst) And (vResult <= dtLast)
   'Rest of macro
   MsgBox "Valid date!"