Avoid int overflow after year 2038

[bmwiedemann]

Avoid int overflow after year 2038, so it can work until year 2106. Note: dates before 1970 will stop working.

See https://en.wikipedia.org/wiki/Year_2038_problem

This patch was done while reviewing potential year-2038 issues in openSUSE.

[mwilck]

Good, but I'd appreciate a more verbose commit message. See comment.

[mwilck]

This needs more explanation in the commit message. AFAIU this is effective because it overrides the standard integral promotion in C, which counter-intuitively promotes unsigned char to int if, as usual, the size of the latter type is larger than the size of the former.

[mwilck]

Extending this with explanation from claude:

• sb is unsigned char[], so each element is unsigned char
• C's integer promotion rules promote unsigned char to int before any arithmetic
• All intermediate computations are therefore signed int
• For timestamps after 2038-01-19 03:14:07 UTC, sb[47] >= 0x80 (the MSB has its high bit set)
• Eventually this means a 1 is shifted into bit 31 of a signed int — this is undefined behavior in C (pre-C23), because left-shifting into the sign bit of a signed integer is UB
• On every real implementation, this produces a negative int
• When assigned to a 64-bit time_t (the type on all modern 64-bit Linux), the negative int is sign-extended to a negative time_t, representing a date before the Unix epoch — completely wrong