[BUG] Claude consistently miscalculates day-of-week, off by one day

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Across multiple conversations on different devices (Claude Code CLI, claude.ai web, iPhone app), Claude consistently gets the day of the week wrong by exactly one day when reasoning from a date. For example, given "The current date is: 2026-02-09" (a Monday), Claude treats it as Sunday, which cascades into downstream errors like identifying "last Friday" as 02/07 (Saturday) instead of 02/06 (Friday).

This isn't a one-off — it happens reliably across conversations, devices, and contexts. It affects any task involving date math: scheduling, determining trading days, seeding database records for specific weekdays, etc.

What Should Happen?

It should have the correct day and date

Error Messages/Logs

Steps to Reproduce

Start a conversation where the system date is a known weekday (e.g., Monday 2026-02-09)
Ask Claude "what day of the week is today?" or ask it to identify "last Friday"
Observe the answer is off by one day
Expected: Monday Feb 9 → last Friday = Feb 6
Actual: Claude treats Feb 9 as Sunday → "last Friday" = Feb 7 (a Saturday)

Note: Claude wrote this bug report at my request because I got tired of the issue.

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

1.260202.5

Platform

Anthropic API

Operating System

Other

Terminal/Shell

Other

Additional Information

_No response_