Question

I’m not sure if this behaviour is intended, but it seems a bit weird to me. I’m using the code from How do you get DateTime.parse to return a time in your time zone?

require 'date'
estHoursOffset = +10 # Brisbane/Australia
estOffset = Rational(estHoursOffset, 24)

With some times, the DateTime that’s returned is a second earlier:

(DateTime.parse("2012-07-15 16:56:00") - (estHoursOffset/24.0)).new_offset(estOffset)
 => #<DateTime: 2012-07-15T16:55:59+10:00 (2456123.788888889,5/12,2299161)> 

But with other times, it seems correct:

(DateTime.parse("2012-07-15 16:16:00") - (estHoursOffset/24.0)).new_offset(estOffset)
 => #<DateTime: 2012-07-15T16:16:00+10:00 (2456123.7611111114,5/12,2299161)>

The program I’m writing only cares about the minutes, which means I’m getting back 16:55 when I want 16:56.

So my questions are;

• Is this intentional? (If so, is it documented somewhere – I haven’t been able to find anything.)
• Is there a simple way of fixing this programmatically? Since I don’t care about seconds,I suppose I could “round up” the DateTimes returned, but it’d be good to know if this could bring up any other problems in edge cases.

Answer 1

This is probably because floating point numbers are imprecise – the 10/24.0 you are subtracting cannot be represented exactly.

If instead of subtracting that float you subtracted a rational, ie Rational(estHoursOffset, 24) then you should be ok