If you want a cart to start at a certain time, such as your ident coming out of the news, or the ads at the start of your show, you can set a timed start in Rivendell.
Add the item as normal:
Double-click in the white area, and the options window will open:
Check the Start at option, and enter the time you want it to start. Change Transition if previous cart ends before... to Stop:
Click OK, and make sure Rivendell is in Automatic mode. Your timed start will show in blue text. If the timed start is the first cart in the log, the yellow countdown timer will show how long until it will automatically start:
(If you have another cart before the one with the timed start, the timer will show how much time you have to fill in order to fit everything before that timed start in, or how much you're over. This calculation can be incorrect if you have changed items around while something is playing.)