I have developed a simple filter for jekyll liquid module, and put the code in _plugins directory. but when I run jekyll the plugin doesn't work.
here is the directory that I run jekyll: https://github.com/alishir/alishir.github.io/tree/dev
here is the plugin code, it return simple string just for test: https://github.com/alishir/alishir.github.io/blob/dev/_plugins/PersianDate.rb