Two functions for creating a timestamp from a number of seconds. One from StackOverflow and mine. I thought I could do better.

periodFormatIso8601.py

def time_format(seconds: int) -> str:
    """
    From: https://stackoverflow.com/a/68321739/543738
    """

    if seconds is not None:
        seconds = int(seconds)
        d = seconds // (3600 * 24)
        h = seconds // 3600 % 24
        m = seconds % 3600 // 60
        s = seconds % 3600 % 60
        if d > 0:
            return '{:02d}D {:02d}H {:02d}m {:02d}s'.format(d, h, m, s)
        elif h > 0:
            return '{:02d}H {:02d}m {:02d}s'.format(h, m, s)
        elif m > 0:
            return '{:02d}m {:02d}s'.format(m, s)
        elif s > 0:
            return '{:02d}s'.format(s)
    return '-'


def periodFormatIso8601(seconds: int | float) -> str:
    """
    My version of the above.  I thought I could make it output
    in ISO 8601 format and do it better.  It needs a little more work.

    P02D — no "T" if no time components
    PT02H — use "T" if there are time components, even without date components
    PT00S — always include seconds component if seconds is 0
    PT00.143S — only include fractional seconds component when needed
    """

    HOURS_PER_DAY = 24
    MINUTES_PER_HOUR = 60
    SECONDS_PER_MINUTE = 60

    if seconds is None:
        raise ValueError('seconds parameter cannot be None.')
    if seconds < 0:
        raise ValueError('seconds parameter cannot be negative.')
    if seconds == 0:
        return 'PT00S'

    seconds, secondsFraction = divmod(seconds, 1)
    seconds = int(seconds)
    d = seconds // (HOURS_PER_DAY * MINUTES_PER_HOUR * SECONDS_PER_MINUTE)
    h = seconds // (MINUTES_PER_HOUR * SECONDS_PER_MINUTE) % HOURS_PER_DAY
    m = seconds % (MINUTES_PER_HOUR * SECONDS_PER_MINUTE) // MINUTES_PER_HOUR
    s = seconds % (MINUTES_PER_HOUR * SECONDS_PER_MINUTE) % SECONDS_PER_MINUTE

    formattedTime = (
            (f'{d:02d}D' if d else '') +
            ('T' if h or m or s else '') +
            (f'{h:02d}H' if h else '') +
            (f'{m:02d}M' if m else ''))
    sf = f'{secondsFraction:.3f}'[1:] if secondsFraction else ''
    formattedTime += f'{s:02d}{sf}S' if s or sf else ''

    return 'P' + formattedTime


def testFunctions(v, functions):
    print(f'v: {v}')
    for function in functions:
        print(function(v))
    print('- ' * 20)


for n in \
        (
                25 * 60 * 60 + 125,
                17 * 60 * 60 + 35,
                3500,
                21,
                0.25,
                3600 + 1 / 7,
                86400.25,
                60,
                0,
        ):
    testFunctions(n, (time_format, periodFormatIso8601))