def _isoweek1monday(year):
    # Helper to calculate the day number of the Monday starting week 1
    # XXX This could be done more efficiently
    THURSDAY = 3
    firstday = _ymd2ord(year, 1, 1)
    firstweekday = (firstday + 6) % 7  # See weekday() above
    week1monday = firstday - firstweekday
    if firstweekday > THURSDAY:
        week1monday += 7
    return week1monday

def _isoweek1monday_new(year):
    # Calculate the ordinal day of the Monday starting ISO week 1.
    # ISO week 1 is defined as the week that contains January 4th.
    jan4 = _ymd2ord(year, 1, 4)
    jan4_weekday = (jan4 + 6) % 7
    return jan4 - jan4_weekday

import timeit
from _pydatetime import _isoweek1monday, _ymd2ord

def _isoweek1monday_new(year):
    jan4 = _ymd2ord(year, 1, 4)
    jan4_weekday = (jan4 + 6) % 7
    return jan4 - jan4_weekday

# Validation results:
# >>> all(_isoweek1monday_new(i) == _isoweek1monday(i) for i in range(2000))
# True

# Timing results:
# >>> timeit.timeit('_isoweek1monday(2000)', globals=globals())
# 0.4805744589539245
# >>> timeit.timeit('_isoweek1monday_new(2000)', globals=globals())
# 0.4299807079951279