From: Len Lawrence <lcl@...>
Date: 2007-08-04T21:40:00+09:00
Subject: DateTime irrationality

I am starting to recode several home applications in Ruby/GTK2 after
16 years of Tcl/Tk and a foray into Ruby/Tk territory.  The simplest
case would be my sidereal time widget which includes an Stime class,
severely pruned below to show the to_s method (which has been altered
to investigate the faulty behaviour described later).

----------------------------------------------------------------

require 'date'

class Stime

  def to_s ( siderealtime )

    z = siderealtime / 24.0
#    h = (24.0 * z).to_i
    c = Date.day_fraction_to_time( z )
    c[3] *= 86400.0
    c[2] += c[3]
    c[3] = nil 
#    c[0] = h
    sprintf( "%02d:%02d:%06.3f", c[0],c[1],c[2] )

  end

end

puts stime.to_s( 9.334 )
puts stime.to_s( 13.603 )
t = 14.603
puts sprintf( "whatever = %06.3f = %s\n", t, stime.to_s( t ) )
puts stime.to_s( 18.765 )

----------------------------------------------------------------

Results:

09:20:02.400
13:36:10.800
whatever = 14.603 = 13:36:10.800   ???
18:45:54.000

I have established by trying a few values within the range 13.999 to
15.000 that the error is confined to the range 14.000 to 14.999.  The
symptoms are the same on two 64-bit machines, one AMD Opteron (2) and
an Intel Core Duo.  It ain't rational.  Anybody ever seen anything
like this?  

If the two commented lines are uncommented the to_s method always
returns the correct hour value in the string.
Date#day_fraction_to_time calls clfloor (which library?) and includes
an argument 1.to_r (to_r from rational.rb).

Len Lawrence