The Calculation Period sets the time window the rule's metrics come from. The options are: Today, Yesterday, Last 3 days, Last 7 days including today, Last 7 days excluding today, Last 14 days and Last 30 days. The difference between the two 7-day options: excluding today uses only days that have already closed (Meta's standard), while including today also adds the partial performance of the current day. For rules that need to react quickly within the day (for example, pausing for spend with no conversions today), use Today. For more stable decisions, longer periods reduce false triggers caused by swings on a single day.