Freebusy in roundcube, particularly the "FIND AVAILIBILITY" popup, is nonfunctional with 32 bit architectures.
The reason is a date-convertion from string to int (and later back to string) where the date is >32bit wide (example: "20160824180000") in file /usr/share/roundcubemail/plugins/calendar/calendar.php. It gets cut to 32 bit, thus broken.

The conversion seems totaly unnecessary.