[fix][libc] fix timegm month overflow calculation

The original overflow logic used __spm[tm_mon] as the current month's
length when normalizing tm_mday. This produced incorrect dates after
crossing month boundaries and left zero or negative days unnormalized.

Use the difference between adjacent cumulative day offsets with
leap-year handling. Normalize overflow in tm_sec, tm_min, tm_hour,
tm_mday, and tm_mon to keep the affected broken-down fields normalized.

Add regression tests for month boundaries, leap years, large offsets,
zero and negative days, negative time fields, combined overflow, and
boundary carry cases.

Close #11686

Signed-off-by: Hui Su <[email protected]>

0 files changed, 0 insertions, 0 deletions