A rolling 30-day window feels sophisticated. It also lets late payers from older trials wash into the current paid rate. Product teams then celebrate a “recovery” that is mostly delayed cheque clearing plus a definition that never closed.
We teach a calendar-aligned window first: people who entered trial in a named week, observed for a named number of days, with late events parked in an appendix. A controller in the United Kingdom can rebuild it from an export. That is the whole trick. Fancy windows are allowed later, privately, once the boring one is trusted.
Students sometimes arrive with a BI tool that defaults to rolling. Switching the public chart feels like a demotion. The Thursday memo is where we make the demotion official: one public number, one experimental number, and a sentence on which decisions may use which.
If your events arrive two days late, say that in the contract. A window that pretends the warehouse is real-time will lie even if the maths is pretty. Product Funnel Analytics is closer to bookkeeping than to theatre.