Relative Date Pattern 'm9'

# Rust (incorrect - wrong date calculation)
```
$  TZ=UTC cargo run -p uu_date -- -d m9 '+%F %T %Z'
  # Output: 2025-12-23 21:00:00 UTC
```

# GNU (correct)
```
$  TZ=UTC /usr/bin/date -d m9 '+%F %T %Z'
  # Output: 2025-12-22 21:00:00 UTC
```

Found with a fuzzer