Subtract n hours from a DateTime in Ruby

Question

I have a Ruby DateTime which gets filled from a form. Additionally I have n hours from the form as well. I\'d like to subtract those n hours from the previous DateTime. (To

Answer 1

EDIT: Take a look at this question before you decide to use the approach I've outlined here. It seems it may not be best practice to modify the behavior of a base class in Ruby (which I can understand). So, take this answer with a grain of salt...

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MattW's answer was the first thing I thought of, but I also didn't like it very much.

I suppose you could make it less ugly by patching DateTime and Fixnum to do what you want:

require 'date'

# A placeholder class for holding a set number of hours.
# Used so we can know when to change the behavior
# of DateTime#-() by recognizing when hours are explicitly passed in.

class Hours
   attr_reader :value

   def initialize(value)
      @value = value
   end
end

# Patch the #-() method to handle subtracting hours
# in addition to what it normally does

class DateTime

   alias old_subtract -

   def -(x) 
      case x
        when Hours; return DateTime.new(year, month, day, hour-x.value, min, sec)
        else;       return self.old_subtract(x)
      end
   end

end

# Add an #hours attribute to Fixnum that returns an Hours object. 
# This is for syntactic sugar, allowing you to write "someDate - 4.hours" for example

class Fixnum
   def hours
      Hours.new(self)
   end
end

Then you can write your code like this:

some_date = some_date - n.hours

where n is the number of hours you want to substract from some_date