Reporting on % of tickets solved within a set period of time

I'm looking for help/a recipe for reporting on how many tickets (or the % of tickets) solved within 30 days and 60 days of being created. My organization has an expectation that 85% of tickets received will be solved within 30 days, and 98% will be solved within 60 days. I have looked into setting up SLAs to make this reporting possible but it doesn't seem to allow me to setup this specific SLA (i.e. ticket must be solved within 30 days of creation).