Takes in a dict of Asset init args and converts dates to pd.Timestamps

def _convert_asset_timestamp_fields(dict_):
    """
    Takes in a dict of Asset init args and converts dates to pd.Timestamps
    """
    for key in _asset_timestamp_fields & viewkeys(dict_):
        value = pd.Timestamp(dict_[key], tz='UTC')
        dict_[key] = None if isnull(value) else value
    return dict_