import math


def date_label(i):
    """
    Helper funtion, label 67 quarters, from 2000q1 to 2016q3.
    Takes current iteration and returns quarter label.
    """
    dlab = '{0}q{1}'
    # count years
    yr = 2000 + math.floor(i/4)
    # count quarters
    qt = i % 4+1

    return(dlab.format(yr, qt))