@start_at.setter<EOL><INDENT>def start_at(self, start_at):<DEDENT>
if start_at is None:<EOL><INDENT>raise ValueError("<STR_LIT>")<EOL><DEDENT>if len(start_at) < <NUM_LIT:1>:<EOL><INDENT>raise ValueError("<STR_LIT>")<EOL><DEDENT>self._start_at = start_at<EOL>
Sets the start_at of this ModelBreak.
RFC 3339; follows same timezone info as `Shift`. Precision up to the minute is respected; seconds are truncated.

:param start_at: The start_at of this ModelBreak.
:type: str