Subtract n hours from a DateTime in Ruby

Question

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