A code snippet to add #beginning_of_financial_year and #end_of_financial_year methods to Rails' DateTime class.

date_time_extension_spec.rb

# spec/extensions/date_time_spec.rb
# PS: I know this spec code isn't particularly DRY or nice. 

require 'spec_helper'

describe DateTime do

  describe "#beginning_of_financial_year" do
    subject do
      DateTime.now.beginning_of_financial_year
    end

    context "current date is before 1 April" do
      before do
        DateTime.stub(:now).and_return(Date.today.change(:month => 3, :day => 30).to_datetime)
      end

      it "should be set to the the beginning of April" do
        subject.day.should eq 1
        subject.month.should eq 4
        subject.hour.should eq 00
        subject.minute.should eq 00
      end

      it "should be set to the previous year" do
        subject.year.should eq Date.today.year - 1
      end
    end

    context "current date is after 1 April" do
      before do
        DateTime.stub(:now).and_return(Date.today.change(:month => 4, :day => 2).to_datetime)
      end

      it "should be set to the 1 April" do
        subject.day.should eq 1
        subject.month.should eq 4
      end

      it "should be set to the current year" do
        subject.year.should eq Date.today.year
      end
    end
  end

  describe "#end_of_financial_year" do
    subject do
      DateTime.now.end_of_financial_year
    end

    context "current date is before 31 March" do
      before do
        DateTime.stub(:now).and_return(Date.today.change(:month => 3, :day => 31).to_datetime)
      end

      it "should be set to the 31st March" do
        subject.day.should eq 31
        subject.month.should eq 3
      end

      it "should be set to the current year" do
        subject.year.should eq Date.today.year
      end
    end

    context "current date is after 31 March" do
      before do
        DateTime.stub(:now).and_return(Date.today.change(:month => 4, :day => 1).to_datetime)
      end

      it "should be set to the end of the 31st of March" do
        subject.day.should eq 31
        subject.month.should eq 3
        subject.hour.should eq 23
        subject.minute.should eq 59
        subject.second.should eq 59
      end

      it "should be set to the next year" do
        subject.year.should eq Date.today.year + 1
      end
    end
  end
end

date_time_extensions.rb

# config/initializers/date_time_extensions.rb

class DateTime
  def beginning_of_financial_year
    if self < DateTime.new(self.year, 4, 1).midnight
      self.change(:day => 1, :month => 4, :year => self.year - 1).midnight
    else
      self.change(:day => 1, :month => 4).midnight
    end
  end

  def end_of_financial_year
    if self < DateTime.new(self.year, 3, 31).end_of_day
      self.change(:day => 31, :month => 3).end_of_day
    else
      self.change(:day => 31, :month => 3, :year => self.year + 1).end_of_day
    end
  end
end