Back in ruby 1.0, date.rb was a fairly simple 228 line file, written by Yasuo Ohba. It stored dates as year, month, and day integers.
This code is a simplified version of what was in ruby 1.0, and I think that other than the choice of 1 as the default for the year, it's not bad.
The date.rb file from ruby 1.0 can actually be run unmodified on ruby 1.8.
Ruby continued to use a modified version of the 1.0 code until ruby 1.6.0 was released.