Database modelling : Creating a model for restaurant application which gives statistics
Save all of the raw ratings (i.e. individual user inputs) with the date on which the rating was recorded.  

Then you can calculate all of your weekly, monthly and yearly statistics based on the raw data each night.  These rolled up statistics should also be dated (with a date range).  In this way you can also keep multiple weeks, months and years so that you can look at trends if you like.

This is often called a data warehouse.  It is a common way of pre-calculating rolled-up statistics for quick and easy reporting.

When you get past the longest of your roll-ups (e.g. 1 year) you can start to throw away your raw transactional data if you're worried about how much space it takes up.