Use the Date property to set the message's date. By default the message's date is the current date. 

The following sample shows how to send an message dated as yesterday:

Dim m As Message
Set m = Runtime1.NewMessage
m.Date = Date - 1
If (0 = m.Send("myaccount@usermail.com", "bul@bulstone.com", "Test", "Hello world!")) Then
    MsgBox "The message was successfully delivered "
End If