For the average person, a few extra clicks to sign up for an API key might seem trivial. For an autonomous AI agent, it’s an insurmountable barrier. This is the core problem DevTools Feed’s latest discovery, OpenWorkdays, aims to solve. It’s not about a flashy new LLM feature; it’s about the fundamental plumbing that makes AI agents useful in real-world, deterministic tasks.
The Calculation Conundrum for Autonomous Agents
The issue boils down to AI’s notorious weakness: precise, rule-based calculations. While LLMs can churn out eloquent prose and even generate code, asking them to reliably calculate business days—accounting for weekends, holidays, and various counting conventions—often results in confident, yet incorrect, answers. A 2025 arXiv study, “Lost in Time: Clock and Calendar Understanding Challenges in Multimodal LLMs,” supports this, noting that model accuracy “diminishes substantially for lesser-known or arithmetically demanding queries.” This isn’t a minor bug; it’s a systemic flaw that breaks automation.
A 2025 arXiv study (“Lost in Time: Clock and Calendar Understanding Challenges in Multimodal LLMs”) found that models do okay on popular holidays — likely because the answer is memorized — but accuracy “diminishes substantially for lesser-known or arithmetically demanding queries (e.g., 153rd day), indicating that performance does not transfer well to offset-based reasoning.”
The industry’s typical solution has been to abstract these calculations to deterministic tools. But what happens when the tool itself is inaccessible to an agent?
The Unseen Wall: Signup Gates for AI
When the original author sought a hosted business-day API, they encountered a common, yet often overlooked, hurdle: authentication. Services like API Ninjas Working Days, workingdays.org, and even timeanddate.com’s paid tiers, all require account creation and API key provisioning. For a human, this is a two-minute process. For an autonomous agent designed to operate without human intervention, it’s a dead end. There’s no inbox to check for confirmation emails, no dashboard to navigate for key generation. The automation simply halts at the signup screen.
This is where OpenWorkdays, a new, MIT-licensed, zero-dependency tool, steps in. It offers a business-day date API with a radically simple premise: no signup, no keys, just anonymous GET requests. This is precisely the kind of accessible, deterministic utility that autonomous agents require to function effectively.
OpenWorkdays: Simple, Focused, and Honest
The API provides three core functions via a single endpoint: adding business days, counting days between two dates, and checking if a specific date is a business day. Users can customize weekend days and provide their own holiday lists via simple URL parameters.
https://openworkdays.vercel.app/api/businessdays?start=2026-05-15&days=5
It’s crucial to understand the deliberate scope of OpenWorkdays. This isn’t a full-featured global time-and-date service. It’s focused on pure UTC arithmetic, meaning no time-of-day, DST, or timezone complications. Furthermore, it eschews built-in country holiday databases, placing the responsibility for holiday calendars squarely on the user. This design choice keeps the service stateless and free from external data dependencies—a smart move for a free, unauthenticated service.
The Agent-Tool Symbiosis is Key
The real innovation here isn’t just the date calculation itself, but the accessibility of the calculation to AI agents. By providing an unauthenticated endpoint, OpenWorkdays eliminates the human-in-the-loop requirement that cripples other services when interacting with automation. This principle extends even further with an MCP (Model Context Protocol) server endpoint, allowing models in compatible environments like Claude Desktop or Cursor to directly invoke the businessdays tool.
The overarching principle is straightforward: deterministic tasks that AI struggles with should be delegated. But the often-unmentioned corollary is that the deterministic tool must be reachable by the agent. An API gated behind a human-centric signup process isn’t a tool for an autonomous agent; it’s a locked box.
OpenWorkdays is refreshingly honest about its limitations. It’s not trying to be the ultimate date library. Its goal is to serve a specific, critical need for developers building autonomous agents: reliable, accessible business-day calculations. If it prevents just one SLA breach due to a miscalculated deadline, it has succeeded. This is a pragmatic, data-driven approach to a growing problem in AI development.
