What version of Desktop are you using? Have you tried using Relative Dates (when you drop Date on filter shelf select "Relative to now")? There is also this KB article which might help: http://www.tableausoftware.com/community/support/kb/count-any-given-time and this example: http://www.tableausoftware.com/community/support/kb/five-recent-days
I love the relative data feature! Works great.