The Bitemporal Timeline
There are two different questions hiding inside every date in your business: when did this fact hold, and when did we find out. Most systems store only the first, and then quietly lose the ability to reproduce anything they ever published.
On 15 July you publish the Q2 exposure report. It goes to the board and to the regulator. In October, someone re-runs the same report, for the same quarter, from the same system — and gets a different number. Nothing was deleted, no record was corrupted, and the query has not changed. What happened?
Show the stored facts as a table
| Reserve | Effective from (valid time) |
Recorded on (transaction time) |
|---|
Reserve development
Development is the gap between the two axes: what we thought the reserve was, versus what it turned out to be. The triangle is built from transaction time — without it there is no triangle, only a current position.
Restatement and audit
"Show me the numbers as we reported them, and as we understand them now." Two legitimate answers to one question. An auditor asking for the first one and hearing "we can only produce the second" is the start of a bad quarter.
The backdated promotion
A raise agreed in March, approved in June, effective from April. Three dates, and payroll, headcount and the org chart each need a different pair. Systems with one date field force somebody to pick, and everyone downstream inherits the choice.
The number that moved
If last quarter's figure changes when you refresh, you have a bitemporal problem and a single-axis store. Not a data-quality problem — the data is fine, you just never kept the second date.
A system that stores only when a fact held can tell you what is true. A system that also stores when you learned it can tell you what you knew, and when you knew it — which is the question every restatement, every audit, and every "why did this number change?" is really asking. Most systems that "lose history" never lost anything. They simply never had the second axis, and nobody noticed until the first time someone asked.