Let's say you want to AUTOMATICALLY delete all "Promotional" emails from your Gmail after 90 days.
Go to Settings -> Filters -> Create a new filter
Type the following into "Has the words" field:
Click "Create filter with this search"
Select "Delete it" and finally click "Create filter" button...
And... we're done!
The key here is older_than:90d, you can just as easily create whatever time-based filter you want with that.
Additionally, the reverse of that filter is newer_than:90d
So, for example, using the same steps as above, we can create a Gmail filter that puts emails newer than 90 days under a specific label/folder... Not sure exactly how this would benefit someone but I'm sure someone will find a use for the filter "newer_than:Xd"