the times aren't in the correct format, have a check of
http://dev.winamp.com/wiki/SHOUTcast...n#Calendar_Tag for the format it needs to be in (handling is pretty strict which is most likely why it's not working in testui).
there is an api to control this which is all in the testui code or via the ajax api details on the wiki / in the sc_trans docs from your install (assuming you're using build 40).
as long as what is being played then that should play for the desired time. however, playlist events are queued up so if the current playlist goes over that time then it'll take until the end of the currently playing song for the event to take effect - this may change in a newer version of sc_trans but i'm not sure at the moment.
-daz