Most real-time clock hardware stores datetime as separate binary-coded decimal fields representing months, days, hours, minutes, and seconds as one byte each, and often the year too (resulting in a year 2100 limit).
Python's datetime, WIN32's SYSTEMTIME, Java's LocalDateTime, and MySQL's DATETIME similarly have separate attributes for year, month, day, etc.
NTFS stores a 64-bit number representing time elapsed since the year 1601 in 100-nanosecond resolution for things like file creation time.
NTP uses an epoch of midnight 1900-01-01 with unsigned seconds elapsed and an unusual base-2 fractional part
GPS uses an epoch of midnight 1980-01-06 with a week number and time within the week as separate values.

Converting between time formats is a common source of bugs and each one will overflow in different ways. A time value might overflow in the year 2036, 2038, 2070, 2100, 2156, or 9999.

Also, Unix time is often managed with a separate nanoseconds component for increased resolution. Like in C struct timespec, modern *nix filesystems like ext4/xfs/btrfs/zfs, etc.