Description of date comparison in WordPress Google Analytics plugin.
Metrics comparison lets you compare two metrics on a single graph, within a specified date range. Whereas Compare Date option measures tracking of one specified metric during two date periods. Click Compare Date button to open date picker.
Previous period. This option automatically sets the previous period of the time frame you selected for Date option. E.g. if you chose Last Week, Previous period option will be set to the week before.
Previous year. Compares the selected period report with the same period of the previous year.
Custom range. Use the calendars to set custom date range for comparison. You can measure statistics of any period with this option. For example, you can set the initial report date range from 22nd of February to 12th of March, 2015, and compare it to 4th to 26th of May, 2016.