Right now, the various functions in utils.data that have a cache option hold onto their cached copy of a download until the file is deleted. There are some use cases that require the cache expire after a certain time, and get re-downloaded (e.g. #1145).

So these functions (which all depend on download_file I believe), should allow cache to give a specific timeframe after which the file should be re-downloaded. This is pretty straightforward, but there is a question how best to define the expiration time. My instinct is to use stdlib datetime.datetime and datetime.timedelta objects, but an alternative might be astropy.time.Time and astropy.time.DeltaTime.