Private Sub cmdOK_Click()
Dim rngStart As Range, rngEnd As Range
Set rngStart = Columns(1).Find( _
what:=DateValue(txtStart.Text), LookIn:=xlFormulas)
Set rngEnd = Columns(1).Find( _
what:=DateValue(txtEnd.Text), LookIn:=xlFormulas)
Columns("B").Clear
Range(rngStart, rngEnd).Offset(0, 1).Value = "X"
End Sub