1. Name the clock-start
A cohort without a stamp is a mood. We force a written choice: first-open, first authenticated session, first successful order, trial start, or first paid period. Mixing two clocks in one Day-14 is how paywall debates go nowhere.
2. Hold the house windows
Proxy Cloudhub uses Day-7, Day-14, and Day-28 until those three survive critique. Custom intervals are allowed only after the house windows are stable. This is not superstition; it is how teams stop inventing a new window every campaign.