Backward Time Step

["Min, Dughong" <min@xxxxxxxxxxxxxx>]

Hello, Ferret Users;

I like to know whether ferret supporst the backward time step.
Using the lagrangian drifter routine designed by Mark Verschell, 
I have found passways for some particles. But for this time, I
want to see where  particles are originated  from . Therefore,
I should use the backward time step way in lagrangian drifter
routine. Are there any way to deal with that question in ferret? 
Any help will be very appreciated. 
Thanks in advance.

Duke Min