def get_cv_object(train):
    """
    Return Time Based CV Object. 
    Time based CV scheme: {train month(s) - test month}
                            {1}-{2}
                            {1,2}-{3}
                            {1,2,3}-{4}
                            {1,2,3,4}-{5,6}    
    """
    tx_month = train['DayNumber']//30
    
    d = {}

    for fold in range(5):
        if (fold == 4):
            break
        elif (fold == 3):
            d['fold_'+str(fold)+'_train'] = tx_month.loc[(tx_month <= fold)].index
            d['fold_'+str(fold)+'_test'] = tx_month.loc[(tx_month > fold + 1)].index
        else: 
            d['fold_'+str(fold)+'_train'] = tx_month.loc[(tx_month <= fold)].index
            d['fold_'+str(fold)+'_test'] = tx_month.loc[(tx_month == fold+2)].index
            
    
    cvobj = [ (d['fold_' + str(fold) + '_train'], d['fold_' + str(fold) + '_test']) for fold in range(4)]
    
    return cvobj