Neither is more difficult than then other - code for both below:
Option Explicit
Sub AddDate()
Dim olItem As MailItem
Dim strDate As String
On Error GoTo lbl_Exit
For Each olItem In Application.ActiveExplorer.Selection
strDate = Format(olItem.ReceivedTime, "yyyy.mm.dd - ")
olItem.subject = strDate & olItem.subject
olItem.Save
Next olItem
lbl_Exit:
Set olItem = Nothing
Exit Sub
End Sub
Sub AddDate2()
Dim olItem As MailItem
Dim olFolder As Folder
Dim strDate As String
On Error GoTo lbl_Exit
Set olFolder = Session.PickFolder
For Each olItem In olFolder.Items
strDate = Format(olItem.ReceivedTime, "yyyy.mm.dd - ")
olItem.subject = strDate & olItem.subject
olItem.Save
Next olItem
lbl_Exit:
Set olItem = Nothing
Exit Sub
End Sub