_increment_date() kept its own copy of the days-per-month table and leap-year check, duplicating _days_in_month() in the same file. Two sources of truth is how the July day-count typo went unnoticed. Call _days_in_month() directly instead, leaving one table in the file. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01PSucp94GA31sshe5AjvSzQ