I've expected to compare a day period with same day period in year before. Two pre-archived reports. So I checked in the calendar-dropdown:

1. 2018-11-05 (day)
2. «compare to:» and «previous year»

I expected to have two day period comparison like happend on «previous period» but worked like «custom», returning a range.

On huge websites, ranged report must be calculated on the fly by Matomo, while day, week, month, year periods are pre-calculated during cron archive.

Then I've expected some missing features:

• week periods comparison between years (for example 43th week of 2018 vs 43th week of 2017)
• month period comparison between years (november 2018 vs november 2017), not ranged from 2017-11-01 and 2017-11-30

The problem with ranged data is the server gets overloaded on big amounts of data. You can feel it by comparing «period year» with «previous year», then «period year» with «previous period». The second (pre archived) is faster.