Subtract n hours from a DateTime in Ruby
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?
You could do this.
adjusted_datetime = (datetime_from_form.to_time - n.hours).to_datetime
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.
The advance method is nice if you want to be more explicit about behavior like this.
adjusted = time_from_form.advance(:hours => -n)
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