Normalize a time to be an int measured in microseconds.

def _NormalizeTime(self, time):
    """Normalize a time to be an int measured in microseconds."""
    if isinstance(time, rdfvalue.RDFDatetime):
      return time.AsMicrosecondsSinceEpoch()
    if isinstance(time, rdfvalue.Duration):
      return time.microseconds
    return int(time)