[BUG] Claude Opus 4 generates incorrect date - shows January instead of August

Environment

• Platform (select one):
• [x] Anthropic API
• [ ] AWS Bedrock
• [ ] Google Vertex AI
• [ ] Other: <!-- specify -->
• Claude CLI version: <!-- Please run claude --version and paste output here -->
• Operating System: macOS (Darwin 24.6.0)
• Terminal: <!-- Please specify your terminal app -->

Bug Description

Claude Opus 4 model consistently converts the actual month to "January" when generating dates. This issue occurs across different interfaces (Claude Code CLI and browser-based Claude), suggesting a model-level date processing bug.

Steps to Reproduce

1. Use Claude Opus 4 model (claude-opus-4-1-20250805)
2. Request generation of content containing current date (e.g., create a filename with date)
3. Observe that month "08" (August) is converted to "01" (January)

Examples:

• Claude Code: Generated filename handover_20250122.md instead of handover_20250822.md
• Browser Claude: Notion integration wrote date as January instead of August

Expected Behavior

Generate dates that accurately reflect the actual current date (August 22, 2025)

Actual Behavior

Consistently generates January (01) instead of the correct month (08), while day and year remain correct

Additional Context

• This issue has occurred multiple times over different sessions
• System date is correct: date command shows "2025年 8月22日"
• Claude Code environment shows correct date: "Today's date: 2025-08-22"
• Issue affects both filename generation and content written to external services
• The pattern is consistent: always converts to January, never to other months
• This appears to be a model-level issue as it occurs across different Claude interfaces