Period number, starting from the first time column

Totals the aggregates and inputs. The first time column is number 0.

The number of time columns in the model since the first period.

True—if the period number equals the number enter in @if

False—if not

@if(@pdnum= 2, 1, 2)

This executes (result: 1) after the formula reaches the third time column in the model.