The week starts on different days between Usage/Rules Engine and MDA

I just came across an odd situation. I was loading some data into MDA (creating zeros so the time series charts look right) and selected "first day of last week" as the date value.

When I checked the file, the "first day of last week" according to the rules engine was 2/27/16, which is right because my Time Granularity under Usage Configuration is set so the week starts on Saturday. (Why Saturday? No idea... That's just what we did back when! :-)

But when I group the data by week in the MDA report I get the week starting on 2/29/16.

It's easy enough to fix in this use case, but it could through some people off so I thought it'd be worth highlighting.