An autotext as suggested by Charles would do the job, or you could use a simple macro
Code:
Sub InsertDateandTime()
Dim orng As Range
Set orng = Selection.Range
orng.ParagraphFormat.Alignment = wdAlignParagraphLeft
orng.Text = Format(Date, "d mmmm yyyy") & vbCr & Format(Time, "hh:mm am/pm")
orng.Collapse 0
orng.Select
lbl_Exit:
Set orng = Nothing
Exit Sub
End Sub
Change the date and time formats as required.