Mike,
The answers depend what type of form and fields that your are using. For a pure field code solution see: DateCalc.zip at
http://www.gmayor.com/downloads.htm
You can also use VBA to adjust dates:
Sub ScratchMacro()
'A quick macro scratch pad created by Greg Maxey
Dim oTbl As Word.Table
Set oTbl = ActiveDocument.Tables(1)
Dim oDate As Date
'Will error if cell text is not a valid date.
oDate = Left(oTbl.Cell(1, 1).Range, Len(oTbl.Cell(1, 1).Range) - 2)
oTbl.Cell(1, 2).Range.Text = DateAdd("d", 60, oDate)
oTbl.Cell(1, 3).Range.Text = DateAdd("d", -15, oDate)
End Sub