15分区切りで、次の時間を知りたいとき

こういうのをやりたい

• 13:00 => 13:15
• 13:05 => 13:15
• 13:21 => 13:30
• 23:47 => 00:00 （次の日)

Active Support を使うとヨサソウ
(日、月、年を跨ぐときもこれでおっけーなはず)

require 'date'
require 'active_support'
require 'active_support/core_ext'

QUARTER_HOUR = 15

def quarter_hour_time(date: DateTime.now)
  date.beginning_of_hour + ((date.min / QUARTER_HOUR + 1) * QUARTER_HOUR).minutes
end

now = DateTime.now
puts now
puts quarter_hour_time(date: now)

nenmatsu = DateTime.new(2017, 12, 31, 23, 55)  
puts nenmatsu                            # => 2017-12-31T23:55:00+00:00
puts quarter_hour_time(date: nenmatsu)   # => 2018-01-01T00:00:00+00:00