My hair on fire rule of metrics

I feel since I talk about this rule a fair amount I should have it published somewhere. A hair on fire rule is one which when noticed you don't wait for arguments to weigh the pros and cons. You just put out the fire. I have one rule like this for metrics.

A metric for a time period can't change after it has been reported.

This doesn't mean you have to be able to report the metric immediately after the time period has ended. And it doesn't mean you can't fix errors later. But it means that the definition of the metric shouldn't be affected by future events.