My understanding is that when you select to create weekly time reporting periods the system allows the maximum of seven days in the period.
If you want some be longer than that you have to create biweekly periods and if they are not exactly 14 days you set the dates manually for each one that is pf different length.
That is if you want to have mostly weekly but some longer and some shorter you create the weekly from the start to finish of the seven day periods. Then you create the next one setting the start and finish. If it is less than 7 days the select weekly if it is longer than 7 but not longer than 14 create biweekly. After that you can create again a bunch of weekly periods.