Sub Add_Date()
Dim c As Range
Dim idate As Long
On Error GoTo error_handler:
idate = Application.InputBox("Enter Date", "Input data", , , , , , 1)
For Each c In Selection
If c.Value <> "" Then c.Value = idate + c.Value
Next
On Error GoTo 0
Exit Sub
error_handler:
MsgBox "Invalid date !", vbCritical
End Sub