How to validate date attributes in Rails(Before_create)?

I have two models...

models/Resident.rb : has_many: leaves

models/leave.rb: belongs_to: resident

Now what I want to validate leave model attributes before they get created..

leave.rb attributes : start_date,end_date,destination

here is my leave model:

class Leave < ActiveRecord::Base
  belongs_to :resident
  validates :destination,presence:true
  validates :end_date,presence: true
  validates :start_date,presence: true
  before_create :check_correct_leave

  private

  def check_correct_leave

   if resident.hostel.hostel=='J'
     (self.end_date - self.start_date).to_i == 4 ||  (self.end_date - self.start_date).to_i == 5

   else
     errors.add(:start_date, "Leave are granted only 4 or 5 days")
   end

  end

end

I want check_correct_leave method to also check --> if the resident already have a leave (stored in a leave model) of that a month (month means jan,feb,etc) then it should generate an error that:

"You can't mark leave cause you have already marked leave for this month" and model should not store that leave. Thanks !