Rubyで時間を15分刻みで丸める ( 切り上げ / 切り捨て )

rounding という gem を使えば楽。

# gem install rounding
require 'rounding'

Time.parse('2018/08/18 0:49 +09:00')
# => 2018-08-18 00:49:00 +0900

# 15分刻みで切り上げ
Time.parse('2018/08/18 0:49 +09:00').ceil_to(15.minutes)
# => 2018-08-18 01:00:00 +0900

# 15分刻みで切り捨て
Time.parse('2018/08/18 0:49 +09:00').round_to(15.minutes)
# => 2018-08-18 00:45:00 +0900

Rounding is compatible with ActiveSupport's Time extensions, but also works fine without ActiveSupport.

ActiveSupport の Time とも互換性があるとのこと。

rounding は時間だけでなく、数値全般を扱える強力な gem なのでとても便利そう。

参考

brianhempel/rounding: Round by any arbitrary step in Ruby.
https://github.com/brianhempel/rounding