Depends on what you expect when mentioning an "alert". If you have expired rules it's going to be mentioned in the task details of a related policy installation.
"Recent tasks" => "Policy Installation - <policy name>" => "Details" => "Status"
e.g.
"Time object xyz in rule xyz has expired.
..."
If you expect to receive a mail or something like that I have to disappoint you. This is - currently - not possible with on-board tools. However it can be achieved via the management api and some manual work. You need to receive all rules from a given rulebase and take note of the ones that contain a time object. After that you need to check the current date and also the dates within the time object and compare them... I guess you get the idea.
Official statement here.
Regards,
Maik
