Google recently announced that on January 4, 2013, they will shut down several less popular Google Calendar features. Among them will be Appointment Slots. As of January 4, you will be unable to create new reservable times on your Calendar through Appointment slots, but existing Appointment slots will continue working for one year. (The M+Google team will research and test third-party alternatives to Appointment Slots and will post an update when more information is available.)
In addition, they will discontinue two Calendar Labs—Smart Rescheduler (we recommend Find a time view or Suggested times as alternatives) and Add gadget by URL. Finally, Check your calendar via sms and Create event via sms (GVENT)—features for creating and checking meetings by texting information to Google—were discontinued as of December 14, as most users prefer mobile Calendar apps.