With AI you could build a ledger from scratch. Here is what you would be building.
| What a ledger must get right | What goes wrong when it does not | Status here |
|---|---|---|
| Value dating: trading date and value date on every movement, every property a date series, back-dated corrections re-accrued | A rate stored as a column; interest wrong by a day, on every account, discovered at month-end | Core, event-sourced, tested |
| Accrual and compounding across day counts and conventions | Rounding drift that never reconciles | Core, five conventions from the ISO code set |
| Day-end that can run forward and replay | Forecasts disagree with what actually posts | One code path, parity tested |
| A sub-ledger that balances per entity and per currency, and reconciles to your general ledger | Manual journals, FX result in the wrong place, a month-end that never ties | Core, posting rules configured, journal export |
| Four-eyes control on the operations that matter | A single clerk moves a limit | Deny by default, authorisation eligibility derived |
| Multi-currency revaluation and period close | A profit that disappears when the rate moves | Core, in the books |
| Schedules on irregular frequencies with multiple calendars | Payments on public holidays | Core, configured per account |
| Structures and legal entities | The deal you administer on a spreadsheet | The reason this engine exists |
Cross-feature references, static handlers and untyped ids fail the build.
A command with no permission entry is a build failure, not a hole.
A handler no test dispatches fails the gate.
Every deviation from a standard is a decision record.
506 of them, the design history of the engine.
A greenfield build gets you a ledger in months and the edge cases over years.
The edge cases are the product. The mistakes surface in month-end runs, audits and customer statements, and each is a data fix on a live book. Twenty-five years in production at one of the world's safest banks produced a handful of true defects. That record is the asset; the code is how it is delivered.
What this means for you: you get the years, not just the code. Your AI works inside the rules those years wrote.