How to round a time down to the nearest 15 minutes in Ruby?

You said “round down”, so I’m not sure if you’re actually looking for the round or the floor, but here’s the code to do both. I think something like this reads really well if you add round_off and floor methods to the Time class. The added benefit is that you can more easily round by any time partition.

require 'active_support/core_ext/numeric' # from gem 'activesupport'

class Time
  # Time#round already exists with different meaning in Ruby 1.9
  def round_off(seconds = 60)
    Time.at((self.to_f / seconds).round * seconds).utc
  end

  def floor(seconds = 60)
    Time.at((self.to_f / seconds).floor * seconds).utc
  end
end

t = Time.now                    # => Thu Jan 15 21:26:36 -0500 2009
t.round_off(15.minutes)         # => Thu Jan 15 21:30:00 -0500 2009
t.floor(15.minutes)             # => Thu Jan 15 21:15:00 -0500 2009

Note: ActiveSupport was only necessary for the pretty 15.minutes argument. If you don’t want that dependency, use 15 * 60 instead.