Make our naive 1.month == 30.days more explicit

Rails 5.1 switched from julian to gregorian calendar so 1.month and
1.year changed in value.

Note,  duration_of_report_step consumers should be 30.days and sometimes 1.month, ugh

rails/rails#27610 (comment)
rails/rails@2a5ae2b
Rails PR: #29971

app/models/chargeback.rb

@@ -136,11 +136,14 @@ def new_chargeback_calculate_costs(consumption, rates)
         data[measure][dimension] = [value, r.showback_unit(ChargeableField::UNITS[r.chargeable_field.metric])]
       end
 
+      # TODO: duration_of_report_step is 30.days for price plans but for consumption history,
+      # it's used for date ranges and needs to be 1.month with rails 5.1
+      duration = @options.interval == "monthly" ? 30.days : @options.duration_of_report_step
       results = plan.calculate_list_of_costs_input(resource_type:  showback_category,
                                                    data:           data,
                                                    start_time:     consumption.instance_variable_get("@start_time"),
                                                    end_time:       consumption.instance_variable_get("@end_time"),
-                                                   cycle_duration: @options.duration_of_report_step)
+                                                   cycle_duration: duration)
 
       results.each do |cost_value, sb_rate|
         r = ChargebackRateDetail.find(sb_rate.concept)

app/models/chargeback/consumption.rb

@@ -18,7 +18,7 @@ def consumed_hours_in_interval
     def hours_in_month
       # If the interval is monthly, we have use exact number of days in interval (i.e. 28, 29, 30, or 31)
       # othewise (for weekly and daily intervals) we assume month equals to 30 days
-      monthly? ? hours_in_interval : (1.month / 1.hour)
+      monthly? ? hours_in_interval : (30.days / 1.hour)
     end
 
     def consumption_start
@@ -45,7 +45,7 @@ def hours_in_interval
 
     def monthly?
       # A heuristic. Is the interval lenght about 30 days?
-      (hours_in_interval * 1.hour - 1.month).abs < 3.days
+      (hours_in_interval * 1.hour - 30.days).abs < 3.days
     end
 
     def born_at

spec/models/chargeback_rate_detail_spec.rb

@@ -103,7 +103,7 @@
     end
   end
 
-  let(:consumption) { instance_double('Consumption', :hours_in_month => (1.month / 1.hour)) }
+  let(:consumption) { instance_double('Consumption', :hours_in_month => (30.days / 1.hour)) }
 
   it '#hourly_cost' do
     cvalue   = 42.0
@@ -169,7 +169,7 @@
     let(:monthly_rate) { FactoryGirl.build(:chargeback_rate_detail, :per_time => 'monthly') }
     let(:weekly_consumption) { Chargeback::ConsumptionWithRollups.new([], Time.current - 1.week, Time.current) }
     it 'monhtly rate returns correct hourly(_rate) when consumption slice is weekly' do
-      expect(monthly_rate.hourly(rate, weekly_consumption)).to eq(rate / (1.month / 1.hour))
+      expect(monthly_rate.hourly(rate, weekly_consumption)).to eq(rate / (30.days / 1.hour))
     end
   end
 

spec/models/chargeback_vm_spec.rb

@@ -1398,3 +1398,30 @@ def find_result_by_vm_name_and_region(chargeback_result, vm_name, region)
     include_examples "ChargebackVm", :skip
   end
 end
+
+describe Chargeback::Consumption do
+  context "#hours_in_month" do
+    before do
+      @start_time = Time.parse("2017-10-1 15:53:55 -0400")
+    end
+
+    [
+      [33.days, 720],
+      [32.days, 768],
+      [31.days, 744],
+      [30.days, 720],
+      [29.days, 696],
+      [28.days, 672],
+      [20.days, 720],
+      [5.days,  720]
+    ].each do |duration, expected|
+      it "next #{duration / 1.day}.days" do
+        expect(described_class.new(@start_time, @start_time + duration).hours_in_month).to eq(expected)
+      end
+
+      it "prior #{duration / 1.day}.days" do
+        expect(described_class.new(@start_time - duration, @start_time).hours_in_month).to eq(expected)
+      end
+    end
+  end
+end