Easily add jquery inputmask to your Formtastic (or ActiveAdmin).
1. Include to your Gemfile
gem 'formtastic-jquery_inputmask'
2. Require javascript dependency
//= require formtastic/inputmask
3. Load locale or other inputmask file as you want
//= require formtastic/inputmask/locales/pt-BR
//= require inputmask/phone-codes/phone-us
p.s. it auto adds jquery as a dependency in your project
f.input :created_at, mask: 'datetime'
f.input :date, mask: { alias: 'datetime', inputformat: 'dd/mm/yyyy' }