Support for DateTimes was not added until ruby 1.8.0. In ruby 1.8.0, the Date and DateTime classes were pretty much the same.
In order to handle fractional days, the date was now stored as a rational, and included a time zone offset, even though dates themselves don't have fractional components or specific timezones.
Basically, in ruby 1.8 and 1.9, Date objects don't really represent dates, they represent DateTimes, usually at midnight in UTC.
The 1.8.0 version of Date was about 3 times slower than the 1.6.0 version, and 4 times slower than the 1.0 version.