Time issue in kibana

Suppose I select an index in kibana : logstash-2016.05.01
Which means that this index contains logs of the date 1st May, 2016.
But after selecting the index, when I go to the discover tab, I still have default logs set to "Last 15 minutes"
If I have to set the time again manually, what is the purpose of indexes?

Indices are logical containers for data.
Having one per day makes retention management easier.