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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 _datetime_to_millis(dtm):
 """Convert datetime to milliseconds since epoch UTC."""

    if dtm.utcoffset() is not None:
        dtm = dtm - dtm.utcoffset()
    return int(calendar.timegm(dtm.timetuple()) * 1000 +
               dtm.microsecond // 1000)