Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add randommonicle/claude-skills --skill date-parse-utc-safegit clone --depth 1 https://github.com/randommonicle/claude-skillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/randommonicle/claude-skills/date-parse-utc-safe)<a href="https://agentmods.dev/skills/randommonicle/claude-skills/date-parse-utc-safe"><img src="https://agentmods.dev/badge/skills/randommonicle/claude-skills/date-parse-utc-safe/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/randommonicle/claude-skills/date-parse-utc-safe"><img src="https://agentmods.dev/badge/skills/randommonicle/claude-skills/date-parse-utc-safe.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00099 | $0.00325 |
| Opus 5 | $0.00049 | $0.00162 |
| Sonnet 5 | $0.00020 | $0.00065 |
| Haiku 4.5 | $0.00010 | $0.00032 |
Grade A, and why
date-parse-utc-safe scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 11d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
What it actually says
Date parsing, UTC-safe
One rule, endlessly rediscovered: new Date("YYYY-MM-DD") parses as UTC midnight. In a UTC
runtime (serverless platforms run UTC) rendered through a western timezone, that is the
previous day — bookings display on the wrong date, day-of-week computes wrong, and the bug
never reproduces on a developer machine in the same timezone as the users.
The rule
Never construct a Date from a bare YYYY-MM-DD string for display or day-of-week.
Decompose to explicit local components:
const [y, m, d] = dateStr.split('-').map(Number);
const date = new Date(y, m - 1, d); // local midnight, not UTC midnight
Test the boundary: a date rendered at local midnight in the runtime's timezone is where the off-by-one appears.
Why
The failure only exists in the deployed runtime's timezone, so it survives local testing by construction. Evidence: ICC LESSONS_LEARNED L-005.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 11d ago First seen · 30 lines · 99 tokens per session scan A 3d42813f8409
date-parse-utc-safe is a skill published in the GitHub repository randommonicle/claude-skills (23 stars, last pushed 4d ago), licensed Apache-2.0. It adds 99 tokens to every session and 325 once invoked, about $0.0005 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.
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