Dates & units

Day counts, weekdays and unit conversions — the arithmetic small models get subtly wrong.

Try it live → — load a model once in the demo, then ask one of the questions below.

What it shows

days_between, weekday and convert_units do in code what a small model does from memory and gets off by one: leap years, the weekday of a date in 1969, 26.2 miles in kilometres. convert_units accepts names as well as symbols (“miles”, “fahrenheit”) because models use both.

The tools file

The arithmetic small models get subtly wrong, done by code instead.

// Dates and unit conversions.

tool('days_between', 'Count the days between two dates written as YYYY-MM-DD.',
  { from: 'string', to: 'string' },
  async ({ from, to }) => {
    const a = Date.parse(from), b = Date.parse(to);
    if (Number.isNaN(a) || Number.isNaN(b)) return { error: 'dates must be YYYY-MM-DD' };
    return { from, to, days: Math.round((b - a) / 86400000) };
  });

tool('weekday', 'Get the day of the week for a date written as YYYY-MM-DD.',
  { date: 'string' },
  async ({ date }) => {
    const d = new Date(String(date) + 'T00:00:00Z');
    if (Number.isNaN(d.getTime())) return { error: 'date must be YYYY-MM-DD' };
    return { date, weekday: d.toLocaleDateString('en-GB', { weekday: 'long', timeZone: 'UTC' }) };
  });

tool('convert_units', 'Convert a value between units of length, weight or temperature: km, mi, m, ft, kg, lb, c, f.',
  { value: 'number', from: 'string', to: 'string' },
  async ({ value, from, to }) => {
    const alias = { kilometers: 'km', kilometer: 'km', miles: 'mi', mile: 'mi', meters: 'm', meter: 'm',
      feet: 'ft', foot: 'ft', kilograms: 'kg', kilogram: 'kg', pounds: 'lb', pound: 'lb',
      celsius: 'c', fahrenheit: 'f' };
    const unit = (u) => alias[String(u).toLowerCase()] ?? String(u).toLowerCase();
    const f = unit(from), t = unit(to), v = Number(value);
    if (f === 'c' && t === 'f') return { value: +(v * 9 / 5 + 32).toFixed(2), unit: 'f' };
    if (f === 'f' && t === 'c') return { value: +((v - 32) * 5 / 9).toFixed(2), unit: 'c' };
    const base = { km: [1000, 'm'], mi: [1609.344, 'm'], m: [1, 'm'], ft: [0.3048, 'm'], kg: [1, 'kg'], lb: [0.45359237, 'kg'] };
    const a = base[f], b = base[t];
    if (!a || !b || a[1] !== b[1]) return { error: 'cannot convert ' + from + ' to ' + to };
    return { value: +(v * a[0] / b[0]).toFixed(3), unit: t };
  });

Each tool(name, description, params, handler) is one callable. The file is plain JavaScript, not a module — no imports, no build step, and the demo lets you edit it and apply it live.

Wire it up

import { NexusChat } from 'toolnexus-web';

// loadForTools checks the model can really call a tool before handing it over.
const chat = await NexusChat.loadForTools({ hub: 'onnx-community/Qwen3-0.6B-ONNX' });
await chat.loadTools('./tools.js');   // the file above, saved as tools.js

chat.on('token', (t) => render(t));
chat.on('toolCall', (call, result) => console.log(call.name, result));

const answer = await chat.chat("How many days are there from 2026-01-01 to 2026-12-25?");

Questions to try

• How many days are there from 2026-01-01 to 2026-12-25?
• What day of the week was 1969-07-20?
• Convert 26.2 miles to kilometers.
• Convert 72 fahrenheit to celsius.

What to expect

Not measured by the harness yet. The answers are exact and easy to check against the tool log.

Going further

Add time zones with Intl.DateTimeFormat, or currency with a rates API. Keep one tool per kind of question — a small model picks well from three names, not from thirty.