How to specify time Spans for Message Trace in wpf Application

I had been working on this report on Message trace in Office365. I would want to give filters for Startdate and enddate from my WPF(C#) application. How can i do that.

i tried the following. When i specify them directly in the URL. they work perfectly fine but when i try to give them from the code, thats where it says value cannot be null,Parameter name: Source. What could be the problem.

ub.Query="$filter=StartDate eq datetime'2015-01-01T00:00:00' and EndDate eq datetime'2015-01-07T23:59:59' &";

 string fullRestURL = Uri.EscapeUriString(ub.Uri.ToString());

ub.Query = "$filter=StartDate%20eq%20datetime%272015-01-01T05:00:00%27%20and%20EndDate%20eq%20datetime%272015-01-11T06:00:00%27%20&";

 string fullRestURL = Uri.EscapeUriString(ub.Uri.ToString());