Using a Formula in Filters for dates

Summary:

We have a custom field that has the last maintenance date and we are trying to create a filter that would only show activities that have a last maintenance date of greater than x number of days. The field comes over as a date in yyyy-MM-dd HH:mm:ss format. I was able to make this work using a defined date... i.e. condition of last maintenance date is less than (dynamic) then I can put in a date and it pulls anything prior to that date.

We want the ability to put a number of days in the condition so that we don't have to update the date each day on the filter for it to properly be maintained.