Since I18n is the Rails core feature starting from version 2.2 you can use its localize-method. By applying the forementioned strftime %-variables you can specify the desired format under config/locales/en.yml
(or whatever language), in your case like this:
time:
formats:
default: '%FT%T'
Or if you want to use this kind of format in a few specific places you can refer it as a variable like this
time:
formats:
specific_format: '%FT%T'
After that you can use it in your views like this:
l(Mode.last.created_at, format: :specific_format)