?
This is actually an artifact of the OData specification from when ProGet was first developed - that was actually the preferred format specified, and NuGet.org didn't support $format=json, so we just followed the OData spec.
Anyway, those are just .NET DateTime Ticks in there (and the +0 just indicates UTC), so in your example it would just be:
new DateTime(636650168208570000, DateTimeKind.Utc)
Of course, if we had it to do over again, it would be something more sensible like ISO-8601.