Where exchange rates come from — and how to build a site people can trust

You want to know how many somoni your 100 dollars are worth. You open a website, it shows a number, and you believe it. But think for a second: why do you believe it? Who said that number is right? From which day is it?

This lesson is about trust: where numbers on a website come from, and the boring, careful decisions that make them worth believing. The example is Kursi Tojik — a site I built that shows the official exchange rates of the Tajik somoni.

Who decides the rate?

Every working day, the National Bank of Tajikistan publishes the official rates: how many somoni one dollar, one euro, one ruble and other currencies cost. It puts them on its website, nbt.tj, also in a form that programs can read.

Commercial banks then set their own rates for buying and selling cash — usually a little different from the official one. The National Bank publishes those cash rates too.

So the honest answer to “where does the number come from?” is simple: from the National Bank, on a specific date. Kursi Tojik does not invent, estimate or type in any number. It copies what the bank publishes and says so.

Rule one: every number has a source and a date

The first version of Kursi Tojik had a “money transfer fee calculator”. It looked useful and its tariffs looked real. But the data file behind it called them illustrative — examples, not real tariffs of real companies. It also had bank rates typed in by hand, which go out of date the next morning.

Both were removed. Not “improved” — removed. A feature that cannot point to a real, current source is worse than no feature, because people make money decisions with it.

The nominal trap

Here is a mistake that is easy to make and hard to notice. For some currencies the bank does not publish the price of one unit, but of ten or a hundred. One Kazakh tenge or one Uzbek sum costs so little that the bank writes “10 KZT = …” or “100 UZS = …”. That number — 10 or 100 — is called the nominal.

If a converter forgets the nominal, it is wrong by ten or a hundred times. Try it with the real official rates of 2 October 2026:

Kursi Tojik always divides by the nominal, and shows “10 KZT” on the board exactly as the bank publishes it — so you can compare it with the source yourself.

When the source goes down

The first version asked the bank's website for the rates on every visit. That sounds fresh and simple. But when nbt.tj was slow or unavailable, Kursi Tojik showed… nothing. Or worse, it could show a zero.

So the site was rebuilt around its own notebook:

1. Every three hours, a scheduled job (a cron job — a timer on the server) visits nbt.tj and copies the new rates into a small database.
2. The website and the Telegram bot read only from that database, never from the bank directly.
3. If the bank's site is down, the database still has the last official rate — with its date.

Switch the bank off and compare the two approaches:

It is like a family that writes down the bus timetable once a day. If the bus company's website breaks, the family still knows when the bus comes — and knows the timetable is from this morning.

nbt.tj ──(every 3 h)──▶ collector ──▶ database ◀── API ◀── website
                                              ◀── Telegram bot

Small details that matter

• “Best” in words, not only in colour. The table of bank cash rates marks the best buying and selling rate with the word “best”. Some people cannot tell green from red; a word works for everyone.
• The date is always on screen. Not hidden in a tooltip — next to the numbers.
• Three languages. Tajik by default, plus Russian and English, because rates matter to everyone in the country and to people sending money from abroad.
• The same trick as lesson 1. The Kursi Tojik bot understands messages like “100 usd” — exactly the step-by-step reading you tried in the first lesson.

Going deeper: how it is built

The whole service runs on Cloudflare's network:

• a Worker — a small program that runs on Cloudflare's servers around the world — serves the API and runs the scheduled collection;
• D1 — a small SQLite database on the same network — stores rates, history and bank cash rates;
• the website is plain TypeScript without a framework, so it loads fast even on a weak mobile connection.

The conversion rule in the code is one line — and it is the line that the nominal trap is all about:

// api/src/money.ts — somoni for one unit
export const perUnit = (rate: Rate) => rate.value / rate.nominal;

Automatic tests run on every change, so a mistake like forgetting the nominal is caught by a test before any visitor sees it.

The takeaway for any data product

Trust is mostly made of boring decisions:

1. Name the source. Every number, every time.
2. Show the date. Data is always “as of” some moment.
3. Remove what you cannot prove. Even if it looks nice.
4. Fail loudly. A clear “could not load” is better than a quiet wrong number.

This is true for exchange rates, prices in a shop, a clinic's schedule or a school's grades.

Check yourself

Little dictionary

• Official rate — the rate the National Bank publishes for a date.
• Cash rate — the price at which a commercial bank buys or sells cash.
• Nominal — how many units of a currency a published rate is for (1, 10 or 100).
• Source — where a number comes from.
• Cron job — a task a server runs on a timetable, for example every three hours.
• Database — the site's organised notebook, where it keeps the data it collected.
• Cache / saved copy — data kept nearby so the site does not need to ask the source every time.

Need numbers people can trust?

Prices, schedules, rates, reports — if your website or bot shows numbers, I can make them sourced, dated and reliable, in Tajik, Russian and English. Live example: kursi-tojik.pages.dev. Write to me: @SimorghDev.