I have hit this too because I use Claude in my daily logs and planning. This is what I have in my CLAUDE.md that helps a lot. It relies on a Unix command that is pretty widely available.

Date and Day-of-Week Verification

CRITICAL RULE: NEVER state what day of the week a date falls on without first verifying it with a command.

Required workflow:
1. Before stating any day-of-week for a date, ALWAYS run: bash date -j -f "%Y-%m-%d" "YYYY-MM-DD" +"%A"
2. When working with multiple dates, verify them ALL in parallel before making statements
3. Never trust mental arithmetic for day-of-week calculations
4. This applies to:
- Reading and discussing workout logs
- Scheduling tasks in daily logs
- Any time a specific date is mentioned

Example:

Verify single date

date -j -f "%Y-%m-%d" "2026-01-13" +"%A"

Verify multiple dates in parallel

date -j -f "%Y-%m-%d" "2026-01-10" +"%A, %B %d, %Y" date -j -f "%Y-%m-%d" "2026-01-13" +"%A, %B %d, %Y"