Just to clarify the question-- you're saying that the time properties of the SvnRevisionRange object are inconsistent with the values originally passed in?
If so, this test would fail:
DateTime startDateTime = DateTime.UtcNow.AddDays(-1);
DateTime endDateTime = DateTime.UtcNow;
SvnRevisionRange range = new SvnRevisionRange(new SvnRevision(startDateTime), new SvnRevision(endDateTime));
Assert.AreEqual(startDateTime, range.StartRevision.Time, "The start times are not equal");
Assert.AreEqual(endDateTime, range.EndRevision.Time, "The end times are not equal");
Run the code above as a unit test and post the results...