UTCDateAndTime is a UTC based implementation of class DateAndTime with one instance variable representing the magnitude of the point in time, and another representing local time zone offset.
I recommend loading with the SAR from http://wiki.squeak.org/squeak/6197, as maintaining this in Montecello is problematic.
After the 4.6/5.0 release, I would like to suggest moving Chronology out of the Kernel package into its own package, so that changes in Chronology can be maintained in Montecello without conflicting with the rest of Kernel.
Spur notes: The Spur image provides a huge performance when loading this set of changes. There are two steps in the loading process in which all DateAndTime instances need to be #becomed into new instances. This is painfully slow in the traditional image (with or without Cog), and it is amazing fast in Spur.
That said, there is a bug in the Spur VM (easily resolved I think) that produces incorrect time values. I'll post note about that on vm-dev. I would not recommend using UTCDateAndTime in Spur yet, other than just to see how amazingly fast it is compared to loading the same thing in a non-Spur image.
Dave