Built from the shop floor, not the calendar.

It stops being fine the moment you have two chairs, a deposit to collect, a bottle of shampoo to sell and a stylist on commission. At that point the calendar is the smallest part of the day, and everything around it — the money, the messages, the stock, the people — is where the work actually is.

VoltsBook was built the other way round. Availability is worked out from six layers at once so a slot is only offered when it can genuinely be honoured. Money is held in whole minor units and never a floating-point number. A booking cannot be double-taken even when two customers press the button in the same second, because the write is serialised and re-checked inside the transaction rather than before it.

None of that is visible on a good day, which is the point. The parts we spend the most time on are the ones nobody notices: a booking refused rather than double-sold, a stock count that is right after a race, an invoice that cannot disagree with the appointment behind it, a reminder that is sent exactly once however many times the job retried.

The first VoltsBook was a plugin. It ran on top of a website, shared a database with whatever else the site was doing, and inherited every one of that site's bad days. When the host was slow, the booking page was slow. When a theme update broke something, the diary broke with it.

It also had the problems that plugins of that shape tend to have. Stock came off by one however many you sold. Gift cards could be spent twice if two people redeemed the same code in the same moment. Prices were floating-point numbers, which is fine until it is a penny out on an invoice somebody has already filed.

So this one was written from scratch rather than patched — same features, none of the code. Every one of those defects was written down first as a thing that must not be possible, then closed by the shape of the system rather than by a check somebody has to remember: the database refuses two appointments in one slot, a redemption is a single conditional write, money never becomes a decimal.

The old version still runs, and the businesses on it are being moved across with their appointment numbers, customers and payment history intact. That was a requirement before a line of the new one was written.