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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 start_at(self, start_at):
 """ Sets the start_at of this Shift. RFC 3339; shifted to location timezone + offset. Precision up to the minute is respected; seconds are truncated. :param start_at: The start_at of this Shift. :type: str """


        if start_at is None:
            raise ValueError("Invalid value for `start_at`, must not be `None`")
        if len(start_at) < 1:
            raise ValueError("Invalid value for `start_at`, length must be greater than or equal to `1`")

        self._start_at = start_at