Problem Statement:
"Basically I want to sort bugs with a filter first on date updated descending and then priority descending. Unfortunately date updated is a time (not a date) and therefore the second sort order has no effect."

Analysis:
To be able to see a list of issues which were worked on for a particular date in order of priority, not priority ordered by the time each issue was updated on a particular date. Put another way:

Suggested Solution and Feature Request:
Allowing the date field to be split in two would allow a sort on either date alone, time alone and date time together. This would make the filtering more powerful and solve the problem presented above.