Sorry to bump an old post but this happens when you use the search function
I just wanted to comment that the proposed solution for op's problem does not really work. I guess he/she just gave up and hasn't returned feedback.
I've tried (for the same purpose - sorting by exact modify time)
{last_modified:format_date(iso)}
as well as other variations such as
{last_modified:'format_date($, "yyyy MM dd, hh mm ss")'}
None works. Only the date seems to be stored in the database, and not the hour / minute /second. They all apear as 00 00 00.