Time#since DST issue

Here is some interesting result with Rails 2.1 (without any specific TimeZone configuration).

LONG STORY SHORT:

Loading development environment (Rails 2.1.0)
>> t = Time.parse("Sun Oct 26 00:30:00 +0200 2008")
=> Sun Oct 26 00:30:00 +0200 2008
>> (t+24.hours)==((t+3.hours)+21.hours)
=> false

DETAILS:

As you will see, adding 3.hours and then 21.hours doesn't give the same result as adding 24.hours around DST change!

I live in Europe, and DST change just happens at 02:00:00 in my TimeZone (which is most of Europe's). As expected:

>> t = Time.parse("Sun Oct 26 00:30:00 +0200 2008")
=> Sun Oct 26 00:30:00 +0200 2008
>> t2 = t+3.hours
=> Sun Oct 26 02:30:00 +0100 2008

As you can see, the "missing" hour is taken into account, which is what you would expect!

Now, let's add 21 more hours:

>> t2 = t2+21.hours
=> Sun Oct 26 23:30:00 +0100 2008

Which is right, too, since 24.hours have actually passed between the two displayed times!

However, now just try the following:

>> t
=> Sun Oct 26 00:30:00 +0200 2008
>> t3 = t+24.hours
=> Mon Oct 27 00:30:00 +0100 2008

Huho! There is something wrong here!

Is there anything I am missing?

I understand that in some situations, you might want to add "mathematical" hours (which means without taking care of DST or anything specific: 01:00 + 03:00 = 04:00 and that's it). And sometimes, you just want to add "real" hours, which means "what time is it when 3 hours have passed in the real life". Both are equally acceptable approaches. However, some kind of mix between the two is clearly a dangerous solution!