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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 elapsed_time(self):
 """To know the duration of the function. This property might return None if the function is still running. """

        if self._end_time:
            elapsed_time = round(self._end_time - self._start_time, 3)
            return elapsed_time
        else:
            return None