I think I would like something like this as well, although I am not sure which timestamps would be useful to store.
I have sometimes applied 'date:' to represent date of creation, and I have been using pseudo-ISO 8601 (year-month-date, which is lexicographically sortable), so like:
date:1992-06
date:2000-01-01
date:2008
And I figured I could parse that inside the program for a system:date>1999-02 or something that would intelligently fill in the midnight-blanks as you suggest, although I haven't actually seen many date tags or yet wanted to search that way, so I haven't put programming time into it. I suspect the gathering of accurate timestamps is difficult to automate, although perhaps that is something future Deviant-Art-like parsers could generate. Most artist websites have an uploaded date somewhere, don't they?
If you would like to parse imageboard upload timestamp into hydrus, you might be able to neatly do it with a script that went:
for filename in directory:
try to parse timestamp from that (142454546115642.jpg)
create ISO 8601 datestring from that (or whatever)
copy/rename that file to be '2000-01-01.jpg'
And then import the files into hydrus and use regex path tagging to create your 4chan_timestamp: namespace or whatever.