gistfile1.rb

# app/models/foo.rb

# Keep in mind, this doesn't even factor in TimeZones.
class Foo
  # Including this line gives your model two virtual 
  # attributes - `date` and `time` - which you can use as
  # methods in your form_for / f.text_field calls, etc
  attr_writer :date, :time
  
  # returns the date component of starts_at, to ensure
  # that your `date` field is pre-populated when using
  # form_for.
  def date
    starts_at.strftime("%Y-%m-%d")
  end
  
  # returns the time component of starts_at
  def time
    starts_at.strftime("%H:%M")
  end
  
  # This ensures that the values in your virtual fields
  # are coalesced into your actual database field
  before_save :save_date_and_time
  
  private
  
  def save_date_and_time
    # replace with whatever logic you need to ensure that
    # date and time are in the correct format. All you really need
    # is to ensure that the value you pass to starts_at 
    # matches YYYY-MM-DD HH:MM:SS.
    starts_at = date + time 
  end
end