At first I thought I was doing something wrong, but now I'm pretty certain it's a problem with Exchange PowerShell.
Let's say I want to do a discovery over a single date (yesterday). If I submit the search from ECP, I put in start and end dates of 4/10/2014 and it'll do as expected, which is to search from 4/10/2014 00:00:00 to 4/10/2014 23:59:59.
However if I do the same thing through PS, with the exact same criteria, it'll do a search from 4/9/2014 18:00:00 to 4/9/2014 18:00:59. Below is the output of the PowerShell session clearly showing this discrepancy (sorry for the crude redacting and markup, but Windows Snipping Tool really sucks).
We are running Exchange 2010 SP3 with no rollups.