Daylight savings time difference with rspec?

Hi all,
I wanted to check this with you. I have an rspec example that checks
the correct expiration date set on an object. The expiration is set in
the model with

expires_on = + next_duration[phase]

where next_duration can either be in units of seconds or n.days (should
be the same thing). n can be 3, 7, 25, or 30 days.

in my example I check an interval because the two events are not
simultaneous between when the model calls and when the rspec
example calls it:

rhs = expires_on < ( + (n +200))
lhs = expires_on > ( + (n -200))
(rhs && lhs).should be_true

I find that if n takes the expires_on across the daylight savings time
change then the model and rspec are off by 1 hour.

Why is there a difference? Are Rails and RSpec referencing different
time objects?

Thanks for your help,