Returns a list of columns that are of type 'datetimetz'. If col_name is specified, returns 
    whether the column in the DataFrame is of type 'datetimetz' instead.
    Parameters:
    df - DataFrame
        DataFrame to check
    col_name - string, default None
        If specified, this function will True if df[col_name] is of type 'datetimetz'

def col_isdtz(df,col_name = None):
    """ Returns a list of columns that are of type 'datetimetz'. If col_name is specified, returns 
    whether the column in the DataFrame is of type 'datetimetz' instead.
    Parameters:
    df - DataFrame
        DataFrame to check
    col_name - string, default None
        If specified, this function will True if df[col_name] is of type 'datetimetz'
    """
    col_list = df.select_dtypes(include = 'datetimetz').columns
    if col_name is None:
        return col_list
    else:
        return col_name in col_list