Filter a list of dates according to a filter spec

For example, suppose you make hourly backups, but you don't want to keep older backups. Then a command like this:

cd /backups && ls | datefilter -print0 | xargs -0 rm 

will make a decent selection of files to remove.

Note that datefilter will try to extract a date (in iso8601-like format) from the input strings; it doesn't actually look at the file dates or anything like that.

By default, it will use the following filter specification:

• keep a file every hour for the past 2 weeks
• keep a file every day for the past 4 weeks
• keep a file every week for the past 3 months
• keep a file every 2 weeks for the past year