the Availability Report did not work for me, so I tried to look up the
calculations behind the scenes and I think I found a bug in

frontends/php/include/services.inc.php line 278:

for($curr = ($period_start - (7*24*36000)); $curr<=$period_end; $curr +=
6*3600)

the term 7*24*36000 does not calculate one week but 10 weeks. I changed it
to read:

for($curr = ($period_start - (7*24*3600)); $curr<=$period_end; $curr +=
6*3600)

and suddenly the report makes sense to me.

Maybe you want to include this into the next Zabbix release.