You can specify an amount of time in seconds and it will delete any cache entries that expire before that. It's functionally equivalent to the expireCache script command:
https://sites.google.com/a/webpagetest.o...xpireCache
That said, it still has some significant issues because it deletes the entries forcing a fresh download, not an if-modified-since check (and it's only implemented for IE right now).
A better solution would be to munge the expires headers as they come in and subtract the time from every resource but it hasn't been a high enough priority to bother with.
Do you know any workaround do simulate a visit after a given amount of time?
A solution is to use a script command like "sleep XX", but it does not scale with large values.