Why 24 December + 1 Australian Business Day Can Equal 29 December

A developer has released AU Agent Utilities, an open-source MCP server that performs deterministic Australian business-day calculations for AI agents. The server exposes four tools covering business-day checks, date arithmetic, day counts, and deadline rolling, and supports all eight Australian states and territories via a remote Streamable HTTP endpoint. The developer argues that rule-based tasks like business-day math should be handled by deterministic tools rather than left to LLM guesswork.

A few days ago I shared AU Agent Utilities, a small open-source MCP server I built for deterministic Australian business-day calculations. Since then I've been thinking less about the tool itself and more about the reason it needs to exist. A good example is this: At first glance, an AI agent could easily assume the answer is Monday 27 December. But in NSW: So the next business day was: Wednesday 29 December 2021 That is exactly the kind of calculation I don't want an AI model guessing from memory. LLMs are useful when a task requires interpretation, reasoning or language. But some tasks have a correct answer based on a defined set of rules. Business-day calculations are one of them. If an agent is calculating: then getting the date wrong by even one day can matter. Instead of asking the model: "What is one NSW business day after 24 December 2021?" the agent can call a deterministic tool. The MCP currently has four tools: check australian business day Checks whether a date is a business day in an Australian state or territory and returns the previous and next business day. calculate australian business date Adds or subtracts business days from a date. count australian business days Counts business days between two dates with explicit start/end inclusion rules. calculate australian deadline Calculates calendar-day or business-day deadlines and can roll a result to the next or previous business day. It currently supports: ACT, NSW, NT, QLD, SA, TAS, VIC and WA. The server runs remotely using Streamable HTTP. MCP endpoint: text https://au-business-day.auagentutilities.workers.dev/mcp