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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def POST_timegrid(self) -> None: """Change the current simulation |Timegrid|."""

        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]