From: Greg Go <plant@...>
Date: 2008-01-07T05:04:07+09:00
Subject: DateTime new_offset unexpected results

Hello, everybody:

I'm new to ruby, but I'm pretty sure this isn't a problem with me.

I'm running ruby 1.8.6 on Windows XP, and using NetBeans as my IDE.

My application reads the rise, transit and set times for the sun at my
location from a spreadsheet. The spreadsheet provides the date and time
as an astronomical julian date that I then use to create a number of
DateTime objects.

I manually check for Daylight Savings Time by creating two more DateTime
objects, one for DST start, the other for DST end as class variables.

I have a method that returns the local time of the event as another
DateTime object created with DateTime's new_offset method.

Here's the code that actually determines whether it's DST or not, and
creates the local time object accordingly:

offset = Rational(-7,24)
if (@jd > @@dstStart) && (@jd < @@dstEnd)
   offset = Rational(-6,24)
end
@ld = @jd.new_offset(offset)

@jd is the julian date of the event.

The problem is this:
For some events, @ld is one hour off.

Here's an example:
@jd.to_s #=> 2008-03-09T13:24:59+00:00
@ld.to_s #=> 2008-03-09T06:24:59-06:00

In my book, 13 - 6 = 7, not 6.

I've tried with different offset values, as well:
@jd.to_s #=> 2008-03-09T13:24:59+00:00
@ld.to_s #=> 2008-03-09T08:24:59-05:00 - Correct

@jd.to_s #=> 2008-03-09T13:24:59+00:00
@ld.to_s #=> 2008-03-09T05:24:59-08:00 - Correct

So, it appears that it's only when the offset is -6.

This is only happening to some events. However, all events are instances
the same class, with the code above.

I've used this base class to do the same thing with lunar events, with
no problem.

This seems like a bug to me.

Any ideas as to what's going on?

Thanks for your help!
Greg
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