Appointments+ plugin causing a timeout on my site

I have an appointments booking page which shows the calendars of 3 available studios that my members can make appointments in. If they want to book a 30 minute session, there is no problem navigating among the days and months of the calendar. If they want to book a 1, 2 or 3 hour session, the site will timeout, and I get an error that says:

Fatal error: Maximum execution time of 30 seconds exceeded in /www/wp-includes/plugin.php on line 177

What is going on? We've been using the plugin on our site for a couple of years. This seems to be a recent problem. Please help!!