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

t = Time.new
rounded_t = Time.local(t.year, t.month, t.day, t.hour, t.min/15*15)

require 'activesupport/core_ext'

class Time
  def round(seconds = 60)
    Time.at((self.to_f / seconds).round * seconds)
  end

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

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

t.min - ( t.min % 15)

time.at_beginning_of_last_fifteen_minutes

t = Time.new
rounded_t = t-t.sec-t.min%15*60

Time.at(t.to_i/(15*60)*(15*60))

t = Time.new
truncated_t = Time.at(t.to_i - t.sec - t.min % 15 * 60)

min / 15 * 15

15 => 15
16 => 15
..
29 => 15
30 => 30

(( min + 7.5 ) / 15).to_i * 15

( min.to_f / 15 ).round * 15

# this is an extension of Ryan McGeary's solution, specifically for Rails.
# Note the use of utc, which is necessary to keep Rails time zone stuff happy.
# put this in config/initializers/time_extensions

require 'rubygems'
require 'active_support'

module TimeExtensions
  %w[ round floor ceil ].each do |_method|
    define_method _method do |*args|
      seconds = args.first || 60
      Time.at((self.to_f / seconds).send(_method) * seconds).utc
    end
  end
end

Time.send :include, TimeExtensions

class Time
  def round(sec=1)
    down = self - (self.to_i % sec)
    up = down + sec

    difference_down = self - down
    difference_up = up - self

    if (difference_down < difference_up)
      return down
    else
      return up
    end
  end
end

t = Time.now                             # => Mon Nov 15 10:18:29 +0200 2010
t.round(15.minutes)                      # => Mon Nov 15 10:15:00 +0200 2010
t.round(20.minutes)                      # => Mon Nov 15 10:20:00 +0200 2010
t.round(60.minutes)                      # => Mon Nov 15 10:00:00 +0200 2010