def _datetime_to_date(arg):
    _arg = parse(arg)
    if isinstance(_arg, datetime.datetime):
        _arg = _arg.date()
    return _arg


# Monday to Monday -> 00:00:00 to 00:00:00   month 1st -  next month 1st
Accumulate covariance estimators for each cluster with a pass through the data.
dataset/ETHPy150Open shinux/PyTime/pytime/pytime.py/_datetime_to_date--crossover with--dataset/ETHPy150Open yahoo/lopq/python/lopq/model.py/accumulate_covariance_estimators