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 get a time range).

DateTime has two methods "-" and "<<" to subtract day and month, but not hour. (API). Any suggestions how I can do that?

Answer 1

You could do this.

adjusted_datetime = (datetime_from_form.to_time - n.hours).to_datetime

Answer 2

You can just subtract less than one whole day:

two_hours_ago = DateTime.now - (2/24.0)

This works for minutes and anything else too:

hours = 10
minutes = 5
seconds = 64

hours = DateTime.now - (hours/24.0) #<DateTime: 2015-03-11T07:27:17+02:00 ((2457093j,19637s,608393383n),+7200s,2299161j)>
minutes = DateTime.now - (minutes/1440.0) #<DateTime: 2015-03-11T17:22:17+02:00 ((2457093j,55337s,614303598n),+7200s,2299161j)>
seconds = DateTime.now - (seconds/86400.0) #<DateTime: 2015-03-11T17:26:14+02:00 ((2457093j,55574s,785701811n),+7200s,2299161j)>

If floating point arithmetic inaccuracies are a problem, you can use Rational or some other safe arithmetic utility.

Answer 3

The advance method is nice if you want to be more explicit about behavior like this.

adjusted = time_from_form.advance(:hours => -n)

Answer 4

You just need to take off fractions of a day.

two_hours_ago = DateTime.now - (2.0/24)

• 1.0 = one day
• 1.0/24 = 1 hour
• 1.0/(24*60) = 1 minute
• 1.0/(24*60*60) = 1 second

Answer 5

n/24.0 trick won't work properly as floats are eventually rounded:

>> DateTime.parse('2009-06-04 02:00:00').step(DateTime.parse('2009-06-04 05:00:00'),1.0/24){|d| puts d}
2009-06-04T02:00:00+00:00
2009-06-04T03:00:00+00:00
2009-06-04T03:59:59+00:00
2009-06-04T04:59:59+00:00

You can, however, use Rational class instead:

>> DateTime.parse('2009-06-04 02:00:00').step(DateTime.parse('2009-06-04 05:00:00'),Rational(1,24)){|d| puts d}
2009-06-04T02:00:00+00:00
2009-06-04T03:00:00+00:00
2009-06-04T04:00:00+00:00
2009-06-04T05:00:00+00:00

Answer 6

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 7

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 8

DateTime can't do this, but Time can:

t = Time.now
t = t-hours*60

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 <<

Answer 9

If I'm allowed to use Time instead of DateTime (There are several ways to translate one to another):

# Just remove the number of seconds from the Time object
Time.now - (6 * 60 * 60) # 6 hours ago

Answer 10

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 11

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.