Year, month, day, hour, minute, and second packed into 64 bits

cal.py

# 00000000 11111111 22222222 33333333 44444444 55555555 66666666 77777777
# YYYYYYYY YYYYYYYY YYYYYYYY YYYYYYYY YYYYYYMM MMDDDDDh hhhhmmmm mmssssss

class cal64:
    __slots__ = ('_cal64')
    _cal64: int

    def __new__(cls, year:int, month:int, day:int, hour:int, minute:int, second:int):
        assert 0 <= year    <= 2**38-1
        assert 1 <= month   <= 12
        assert 1 <= day     <= 31
        assert 0 <= hour    <= 23
        assert 0 <= minute  <= 59
        assert 0 <= second  <= 60
        self = object.__new__(cls)
        i = 0
        i |= (0b111111 & second)
        i |= (0b111111 & minute) << 6
        i |= (0b11111  & hour)   << 6 + 6
        i |= (0b11111  & day)    << 6 + 6 + 5
        i |= (0b1111   & month)  << 6 + 6 + 5 + 5
        i |= (0b11111111111111111111111111111111111111 & year) << 6 + 6 + 5 + 5 + 4
        self._cal64 = i
        return self

    @property
    def year(self):     return (0xffffffff_fc000000 & self._cal64) >> 26
    @property
    def month(self):    return (0x00000000_03c00000 & self._cal64) >> 22
    @property
    def day(self):      return (0x00000000_003e0000 & self._cal64) >> 17
    @property
    def hour(self):     return (0x00000000_0001f000 & self._cal64) >> 12
    @property
    def minute(self):   return (0x00000000_00000fc0 & self._cal64) >> 6
    @property
    def second(self):   return (0x00000000_0000003f & self._cal64) >> 0

    def __repr__(self):
        return (
            f'{self.__class__.__module__}.{self.__class__.__name__}'
            f'({self.year}, {self.month}, {self.day}, {self.hour}, {self.minute}, {self.second})'
        )

if __name__ == '__main__':
    import time
    print(cal64(*time.localtime()[:6]))
    print(cal64(10_000_000_000, 12, 31, 23, 59, 59))

dim64.py

#               <---     leap year     --->         <---    normal year    --->
#               De_NoOcSeAu_JuJuMaAp_MaFeJa         De_NoOcSeAu_JuJuMaAp_MaFeJa
DIM64 = 0b00000011_10111011_11101110_11011100_00000011_10111011_11101110_11001100

def dim64(year:int, month:int) -> int:
    """Return number of days in the month (1-12) for a given year."""
    is_leap = year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)
    #not_leap = year % 4 or (year % 100 == 0 and year % 400)
    lshift = month * 2 + is_leap*32
    return 28 + (0b11 & (DIM64 >> lshift))


#             Dec   Nov   Oct   Sep   Aug   Jul   Jun   May   Apr   Mar   Feb   Jan
DIM65= 0b0000_11111_11110_11111_11110_11111_11111_11110_11111_11110_11111_11100_11111

def dim65(year:int, month:int) -> int:
    """Return number of days in the month (1-12) for a given year."""
    is_leap = year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)
    lshift = (month - 1) * 5
    return (0b11111 & (DIM65 >> lshift)) + (is_leap and month == 2)


#               De NoOcSeAu JuJuMaAp MaFeJa
DIM32 = 0b00000011_10111011_11101110_11001100

def dim32(year:int, month:int) -> int:
    """Return number of days in the month (1-12) for a given year."""
    is_leap = year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)
    lshift = month * 2
    return 28 + (0b11 & (DIM32 >> lshift)) + (is_leap and month == 2)

#[1, 0, 1, 0, 1, 1, 0, 1, 0, 1, -2, 1]
#            DNOSU_JJMAMFJ
DIM16 = 0b00010101_10101010
def dim16(year:int, month:int):
    """Return number of days in the month (1-12) for a given year."""
    not_leap = (year % 4 or year % 100 == 0 and year % 400) > 0
    return 30 + (1 & (DIM16 >> month)) - (month==2) - (month==2 and not_leap)


def dim0(year:int, month:int) -> int:
    """Return number of days in the month (1-12) for a given year."""
    is_leap = year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)

    # month is 1-12, just the 4 lowest bits vary - labelled a, b, c, d.
    # result is 28-31, only the 2 lowest bits vary - labelled x, y.
    # In a non leap year (a, b, d) determine (x, y) by the boolean equations
    #    x = a | b | d          y = ~a & d | a & ~d
    # The truth tables and simplified equations circuits can be seen at
    # http://32x8.com/sop4_____A-B-C-D_____m_1-3-4-5-6-7-8-9-10-11-12_____d_0-13-14-15_____option-0_____999781975371824592721
    # http://32x8.com/sop4_____A-B-C-D_____m_1-3-5-7-8-10-12_____d_0-13-14-15_____option-0_____989781966578850595664
    a, b, d = bool(month & 8), bool(month & 4), bool(month & 1)
    return 0b11100 | (a|b|d)<<1 | (~a&d|a&~d) | (is_leap and month == 2)


if __name__ == '__main__':
    import datetime
    import itertools
    years = [0, 1, 2, 3, 4, 100, 400, 2000, 2020, 2021, 2024, 2100, 20000, 20001]
    months = [1, 2, 3, 4, 5, 6, 7, 8, 9, 12]
    years_months = list(itertools.product(years, months))
    for y, m in years_months:
        assert datetime._days_in_month(y, m) == dim64(y, m), f'dim64({y},{m})={dim64(y,m)}'
        assert datetime._days_in_month(y, m) == dim65(y, m), f'dim65({y},{m})={dim65(y,m)}'
        assert datetime._days_in_month(y, m) == dim32(y, m), f'dim32({y},{m})={dim32(y,m)}'
        assert datetime._days_in_month(y, m) == dim16(y, m), f'dim16({y},{m})={dim16(y,m)}'
        assert datetime._days_in_month(y, m) == dim0(y, m), f'dim0({y},{m})={dim0(y,m)}'

$ python3 cal.py
__main__.cal64(2023, 6, 4, 20, 57, 19)
__main__.cal64(10000000000, 12, 31, 23, 59, 59)