Creating retention report in Cognos 10

Question

I have to develop a retention report. I have a requirement where i have to calculate number of active user count at end of each month. Table 1 stores the user details. And table 2 stores the status of the user and the status date. So i get the status as 'Active', 'Inactive' from table 2 by joining these tables on user id. Now user has option to select a month for which he wants to see the counts. And there is another prompt where user can enter the duration (in months) for which the users are active. Suppose user chose Jan 2016 and duration as 4. It should be a crosstab. Row will have 'January'. Columns will be 1, 2, 3, 4 as in no. of users active by jan 31st will come under age 1, no. of users still active by Feb end will be under age 2 and so on. Note that the users active in Jan may also be active in Feb, so counts are not distinct across the row.