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

I like using the helpers in active_support. It makes it really clean and easy to read.

See the example below:

require 'active_support'

last_accessed = 2.hours.ago
last_accessed = 2.weeks.ago
last_accessed = 1.days.ago

There might be a way to use that kind of syntax to do what you are looking for, if the current date is used.

Answer 2

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...

---

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

Answer 3

You can use this :

Time.now.ago(n*60*60)

For example Time.now.ago(7200) will give the date and time that was before 2 hours from now.

Answer 4

You didn't say what use you need to make of the value you get, but what about just dividing your hours by 24 so you're subtracting a fraction of a day?

mydatetime = DateTime.parse(formvalue)
nhoursbefore = mydatetime - n / 24.0

Answer 5

If you are working in Rails, the following super-intutive possibility exists:

> Time.now - 12.hours
=> 2019-08-19 05:50:43 +0200

(This also works with seconds, minutes, days, and years)

if you're using just Ruby, DateTime can't do this, but Time can:

t = Time.now
t = t - (hours*60**2)

Note that Time also stores date information, it's all a little strange.

If you have to work with DateTime

DateTime.commercial(date.year,date.month,date.day,date.hour-x,date.minute,date.second)

might work, but is ugly. The doc says DateTime is immutable, so I'm not even sure about - and <<