From: Walton Hoops <me@...>
Date: 2013-07-03T13:22:28+09:00
Subject: Re: How can I add minutes,seconds and hours to a DateTime object?

On 7/2/2013 4:33 PM, Mike Stok wrote:
> On 2013-07-02, at 3:41 PM, Love U Ruby <lists@ruby-forum.com> wrote:
>
>> finally I came up with as below :
>>
>> require 'date'
>>
>> text_t = '2:12:03 PM ET'
>> dt = DateTime.parse(text_t, '%I:%M %p %Z')
>> # => #<DateTime: 2013-07-03T14:12:03+00:00
>> ((2456477j,51123s,0n),+0s,2299161j)>
>> dt.to_s
>> # => "2013-07-03T14:12:03+00:00" # !> invalid offset is ignored
>> dh = Date._strptime(dt.to_s,'%Y-%m-%dT%H:%M:%S%z')
>> # => {:year=>2013,
>> #     :mon=>7,
>> #     :mday=>3,
>> #     :hour=>14,
>> #     :min=>12,
>> #     :sec=>3,
>> #     :zone=>"+00:00",
>> #     :offset=>0}
>>
>> dh[:hour] += 30 # => 44
>> dh
>> # => {:year=>2013,
>> #     :mon=>7,
>> #     :mday=>3,
>> #     :hour=>44,
>> #     :min=>12,
>> #     :sec=>3,
>> #     :zone=>"+00:00",
>> #     :offset=>0}
>>
>> dh.values[0..-2].join(" ")
>> # => "2013 7 3 44 12 3 +00:00"
>> DateTime.ordinal(*dh.values[0..-3])
>> # => #<DateTime: 2013-01-07T03:44:12+00:00
>> ((2456300j,13452s,0n),+0s,2299161j)>
>>
> ratdog:tmp mike$ pry
> [1] pry(main)> require 'date'
> => false
> [2] pry(main)> dt = DateTime.parse('2:12:03 PM ET', '%I:%M %p %Z')
> => #<DateTime: 2013-07-02T14:12:03+00:00 ((2456476j,51123s,0n),+0s,2299161j)>
> [3] pry(main)> dt + (30.0 / 24) # days
> => #<DateTime: 2013-07-03T20:12:03+00:00 ((2456477j,72723s,0n),+0s,2299161j)>
>
Both approaches make me nervous.  The first because your parsing a date 
where the time is 44:00 (clearly an invalid date), which at best I 
believe is undocumented behavior, and more likely is exploiting a bug 
that may be fixed at a later date.

The is Mike's is probably fine, but makes me worry about rounding errors 
for some values.  Instead I'd do the math using Time, which uses seconds 
instead of fractions of days:

irb(main):006:0> now = DateTime.now.to_time
=> 2013-07-02 22:09:19 -0600
irb(main):007:0> hours = 2
=> 2
irb(main):008:0> minutes = 5
=> 5
irb(main):010:0> now + (hours * 60 * 60) + minutes * 60
=> 2013-07-03 00:14:19 -0600

Ideally the Stdlib would have a TimeSpan class which could be added and 
subtracted from DateTime, or be the result of subtracting 2 DateTimes.  
Would be a good idea for a gem.