code: def POST_timegrid(self) -> None:
          
          init = hydpy.pub.timegrids.init
          sim = hydpy.pub.timegrids.sim
          sim.firstdate = self._inputs['firstdate']
          sim.lastdate = self._inputs['lastdate']
          state.idx1 = init[sim.firstdate]
          state.idx2 = init[sim.lastdate]
docs: Change the current simulation |Timegrid|.