date_time.unit.day is a 5-bit bitfield (max 31). Incrementing 31
directly in the bitfield wrapped to 0 via truncation before the
day > days_in_month bounds check ever ran, so the overflow was
never caught. The result decoded as "day 0" of the current month,
i.e. the last day of the previous month, silently rolling the
displayed date backwards (e.g. Aug 31 -> Jul 31 -> Jun 30).
Compute the incremented day in a plain uint8_t local first, compare
it against the month length, and only then write the final value
into the bitfield.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSucp94GA31sshe5AjvSzQ
Konrad RieckandClaude Sonnet 5
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