62419: Make DateTime not use Xojo.Core.Date class

Feedback Case Number: 62419

While debugging a new app for memory leaks, I listed all the classes and wondered why we got so many date and time zone objects. The class in question only used DateTime for a time stamp.

But for every class we create, we got 4 additional objects for one DateTime.
Looks like DateTime is implemented as a layer on top of Xojo.Core.Date. So for every DateTime, you get a timeZone object, a Xojo.Core.Date and a Xojo.Core.TimeZone object.