First stabilize the wormhole.  Then tow one mouth of the
wormhole through deep space so that it comes very close to a
dense object such  as  a  neutron star.  The star's enormous
gravity  well  would  then  produce  massive  distortions in
space-time  and  eventually  open  up  a  time tunnel.   The
gravity forces  involved would cause  a  discrepancy between
the time experienced  by  the  mouth of the wormhole next to
the star and the exit mouth left elsewhere far away from the
gravity well.

    After generating a time difference  between the two ends
of the wormhole (which  could  take  you  backward  for  any
number of days,  weeks,  months, or years depending upon how
long you left it  next  to  the neutron star)  the  wormhole
mouth can be towed away.   Parked wherever you needed to use
it--perhaps  near the moon to make it easily accessible from
Earth--you would now have a reusable time machine with a set
period of time transfer involved.