lihtness/gnomon-mcp

gnomon-mcp

The pointer on a sundial that turns shadow into time.

A small MCP server for the boring-but-essential utilities every model needs: dates, calendars, arithmetic, unit conversion. Use it so your assistant stops "next-token guessing" math and date math.

Why

LLMs are bad at arithmetic and date math by default. They produce plausible answers that are often wrong by a small amount — exactly the kind of mistake that's hard to notice in a long response. gnomon-mcp exposes deterministic Python implementations through MCP so your model can compute instead of guess.

When an agent should reach for gnomon

Anywhere the next plausible token is not the right answer. Concretely:

• Math that matters — anything beyond trivial mental arithmetic, anything with a decimal point, anything that compounds. Call calc.
• "What day is it" / "how long until" / "how long since" — the model's training cutoff is not today. Call now for a snapshot; calendar with until/since/diff for elapsed time; parse for natural-language dates ("next thursday").
• Date arithmetic across month/year boundaries — adding 30 days, finding a quarter-end, counting business days. Models routinely off-by-one these. Call calendar with add / business_days.
• Unit conversion — call calc_convert. Never eyeball "kg → lb" or "°C → °F".
• Table-row workloads — when the same kind of computation needs to run on every row of a table, both batch tools (calendar, calc) take a list and return a list in order. One call, N results.