Revert the change to 'end_time' parameter
Revert the change to 'end_time' parameter

I understand now that if `start_time` = `end_time`, the model will
run for one time step. This partially reverts the changes in
6d65ec8e72444eb697e6e232aadf05b31e3aad07.

permamodel/permamodel,permamodel/permamodel