I've been struggling with this for a utility that I'm writing for a client, so your question is quite timely for me. Looking at reports in QueryBuilder, I'm seeing that there are two SI_STARTTIME and two SI_ENDTIME values - one set in the SchedulingInfo and the other on the InfoObject itself. The set in SchedulingInfo are the start and end dates for the whole schedule and these are what is returned in your code. The set in InfoObject appears to be the correct date/times for this instance of the report, so those are the fields that you need to use. Unfortunately, there is no "getter"for these properties in the IInfoObject interface. So, the easiest way to get them is going to be to get them from the properties bag. You would do this through something like:
IInfoObject.Properties.get("SI_STARTTIME")
I'm working with this code this morning, so I'll let you know if I find that this doesn't work like I think it will.
-Dell