# How does the iCal synchronization work?

iCal is a handy format to exchange calendars. It is used by:

- Agenda/Calendar applications : iCal, Google Calendar, Outlook Calendar…
- Booking platforms and OTA: AirBnb, booking.com, TripAdvisor, Expedia, VRBO, HomeAway, etc…

What is called iCal synchronization is in fact an import of the iCal feed of the calendars that are “synced” made at a regular frequency by each calendar/booking software. Here is what happens:

- Every hour (or at a custom frequency set in **HBook > iCal sync > iCal sync settings**), HBook checks the iCal feeds from all the calendars you have entered in **HBook > iCal sync > Synchronized calendars**. You can refresh the calendars manually by clicking on “Update calendars” button. Any new reservation will be imported into HBook and appear in the list of reservation. 
- At their own frequency, the external calendars (AirBnb, TripAdvisor, booking.com, HomeAway, Expedia, VRBO, etc…) do the same and check the HBook iCal feeds you have entered in their iCal sync settings. They will import the event (Google or iCal or any calendar software) or block the dates (AirBnb, TripAdvisor, booking.com, HomeAway or any booking platform). Some calendar software or booking platform have a refresh button/link to run the “sync”.