function time() will give you the number of seconds passed since 1/1/1970 (the unix euqivalent of today).
function mktime(hour, minutes, seconds, month, day, year) will give you the number of seconds passed between 1/1/1970 and the given date and time.
Now, by entering the following expression in the relevance field of a question, the question will only be shown from february 20, 2013.
You can of course adjust this to your taste AND similar expressions together etc...
Hope this helps!