Backdating

Backdating means entering a cost with an effective date earlier than today, so it prices the days from that date forward. It is the one intended way past figures change: a supplier increase discovered late is applied from when it really started, and the affected days are recomputed.

It is deliberately narrow. Saving a cost normally closes the previous rule the day before and opens a new one, so nothing before the new date moves. Backdating widens that window on purpose, and only as far back as the date entered.

The recomputation that follows is a feature rather than a repair. Days from the backdated date forward are re-priced with the corrected rule, and everything earlier keeps the value that was effective then, which is what keeps the history a record rather than a rolling estimate.

The habit worth having is to note why. A month that changed after it closed should always have an explanation attached, and a backdated cost is the only explanation that ought to be available.