SECONDS_PER_UNIX_DAY = 24 * 60 * 60
DAYS_PER_MONTH = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]


def is_leap(year):
    return ((year % 4) == 0 and not (year % 100) == 0) or (year % 400) == 0


def in_month(epoch, month):
    assert epoch > 0, "Unix time is only defined after 1.1.1970, 00:00:00 UTC"
    assert 1 <= month < 12, "Invalid month"
    year = 1970
    epoch_of_year = 0

    while True:
        cur_year = (366 if is_leap(year) else 365) * SECONDS_PER_UNIX_DAY
        if cur_year + epoch_of_year > epoch:
            break
        year += 1
        epoch_of_year += cur_year

    days = epoch - epoch_of_year
    _mos = list()
    _mos[:] = DAYS_PER_MONTH[:]
    if is_leap(year):
        _mos[1] += 1
    month_start = sum(_mos[:month - 1]) * SECONDS_PER_UNIX_DAY
    month_end = sum(_mos[:month]) * SECONDS_PER_UNIX_DAY
    return month_start <= days < month_end


def which_month(epoch):
    for month_no in range(1, 13):
        if in_month(epoch, month_no):
            return month_no
    raise Exception("This should never happen")


def in_may(epoch):
    return in_month(epoch, 5)


for ts, may, mo, _ in (
        (1178209938,  True, 5, "# Thu May 03 2007 16:32:18 GMT+0000  # květen nepřestupný"),
        ( 957371538,  True, 5, "# Wed May 03 2000 16:32:18 GMT+0000  # květen přestupný"),
        (1238776338, False, 4, "# Fri Apr 03 2009 16:32:18 GMT+0000  # nekvěten nepřestupný"),
        (1207240338, False, 4, "# Thu Apr 03 2008 16:32:18 GMT+0000  # nekvěten přestupný")
):
    assert in_may(ts) == may
    assert which_month(ts) == mo