Based on followed documentation : 

https://www.zabbix.com/documentation/current/en/manual/it_services/sla

Expected behavior in case of adding downtime in the future and in the past:  SLI should be corrected in accordance of downtime periods. 

 Based on my experiments:

Today: 23.11 at 9:01 I can see that service in "not OK" status from the beginning. 

I've tried to add planned downtime in the past - result:

Downtime reduced for 3 hours but SLI was not changed. See below:

In cease of excluded downtime in future - result the same:

If my understanding correct daily SLI calculation should be like this:

(("Period from 00:00" - ("Not OK period" - "excluded downtimes"))/"Period from 00:00")*100%

If so - this is bug. Please check and let me know.