Performance benchmarks compare the metrics from the selected time period to the immediately preceding, non-overlapping period of the same amount of time. Consider these following examples:
A single day, in this case ‘Wednesday, May 30, 2018’ (highlighted in blue) is compared to the four immediately previous Wednesdays (highlighted in orange):
A custom date range, like this period of eight days, Wednesday May 23 through Wednesday, May 30 2018 (again, in blue), would be benchmarked against May 9 through May 16 (in orange), because it is the most immediately preceding period of Wednesday through Wednesday without overlapping:
Finally, selecting last month takes the previous calendar month, in this case May 2018, takes the total number of days in that month, and benchmarks it against the previous non-overlapping period of the same number of days.
For example, May 2018 contains 31 days, beginning on a Tuesday. So the comparison benchmark period considers the previous 31 days, beginning on a Tuesday so the 31-day period doesn’t overlap. In this case, those 31 days begin on Tuesday March 27.