I've often wondered what the purpose of this (assets_last_updated) Assets config key is. I can see the strtotime() version of it is appended to the cache file, but how does this alter cache behaviour over and above the md5() name? Is it intended to be dynamic (eg one could set it to be a week in advance of "now")? Or is it a way of "versioning" assets? How was it intended to be used?