Free/Busy times are not in UTC
a new appointment with UTC+1 time zone at 09:00 shows as
FREEBUSY;FBTYPE=BUSY:20200312T090000/20200312T093000
in the published free/busy file. This isn't UTC, but should be according to https://tools.ietf.org/html/rfc5545#section-3.8.2.6
Tested with evolution 3.34.4 and 3.36.0