Attached is a sample workbook.
The goal is that we need to calculate the days difference between the date from one row (where action >0) and another row. If we just wanted a datediff between the rows, that's do-able using the regular aggregated functions, or if we wanted to return a value for a certain row then we could do that using an aggregated or non-aggregated function. But to do both at once, we need a table calc.
Here's a table calculation that gets the desired # of days:
IF ATTR([action]) > 0 THEN
The calc requires that the userid, date, and action be in the level of detail in the view. The table calc is set with an Advanced... Compute Using on date, userid, and action, sorted on date/Min/Ascending. I created two different views, one is a simple crosstab and the other being histogram-type view with a a Count of how many userids per days.
Table calc for the win once again - thanks for this precise answer !