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

> Source: <https://dev.to/creatorhub121/why-24-december-1-australian-business-day-can-equal-29-december-59hm>
> Published: 2026-09-15 11:28:57+00:00

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
```