[#5203] date.rb: DateTime.sec_fraction and day_fraction_to_time return incorrect second fraction

I'm following up to the mailing list directly since Tom Copeland advised me
that comments on the Trac bugs don't automatically get posted ... I reopened
the following bug:

http://rubyforge.org/tracker/?func=detail&atid=1698&aid=5203&group_id=426

My repro case is:

I expect the following ruby snippet to print out "1/2"
not "1/172800"

C:\>irb
irb(main):001:0> require 'date'
=> true
irb(main):002:0> t = DateTime.parse('2006-08-03 14:22:44.500')
=> #<DateTime: 424042749929/172800,0,2299161>
irb(main):003:0> puts t.sec_fraction
1/172800
=> nil
irb(main):004:0>

EXPECTED: a Rational equal to 1/2
RECEIVED: a Rational equal to 1/172800

Thanks,
Dave