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.

I am not very familiar with the syntax of ruby but you can round down to the nearest 15 minutes using modulo. (i.e. x - (x modulo 15)). I would guess the syntax would be something like

t.min - ( t.min % 15)

This will make your set of possible values 0, 15, 30, and 45. Assuming 0 <= t.min <= 59.

Since Ruby allows arithmetic (in seconds) on Times, you can just do this:

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

You could do:

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

Chuck's answer, while elegant, will run you into trouble if you try to compare values derived in this way; the usecs are not zeroed out.

Shalmanese' answer takes care of that, or Chuck's can be modified as:

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

Your current evaluation using

min / 15 * 15 

is only truncating the min, so

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

Which is not 'rounding'.

You can approximate rounding in a bad-way with

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

Or, using internals:

( min.to_f / 15 ).round * 15

If you're using ActiveSupport, then you can use some of its time helpers.

t = Time.now
t.at_beginning_of_hour + (t.min - t.min % 15).minutes