Add an in_calendar_months feature

README.md

@@ -59,6 +59,9 @@ TimeDifference.between(start_time, end_time).in_years
 TimeDifference.between(start_time, end_time).in_months
 => 12.0
 
+TimeDifference.between(start_time, end_time).in_calendar_months
+=> 12.0
+
 TimeDifference.between(start_time, end_time).in_weeks
 => 52.14 
 

lib/time_difference.rb

@@ -18,6 +18,10 @@ def in_years
   def in_months
     (@time_diff / (1.day * 30.42)).round(2)
   end
+
+  def in_calendar_months
+    months_between(@start_time, @end_time)
+  end
 
   def in_weeks
     in_component(:weeks)
@@ -82,10 +86,13 @@ def humanize
   private
 
   def initialize(start_time, end_time)
-    start_time = time_in_seconds(start_time)
-    end_time = time_in_seconds(end_time)
+    @start_time = start_time.to_time
+    @end_time = end_time.to_time
+
+    start_time_s = time_in_seconds(start_time)
+    end_time_s = time_in_seconds(end_time)
 
-    @time_diff = (end_time - start_time).abs
+    @time_diff = (end_time_s - start_time_s).abs
   end
 
   def time_in_seconds(time)
@@ -95,5 +102,18 @@ def time_in_seconds(time)
   def in_component(component)
     (@time_diff / 1.send(component)).round(2)
   end
+
+  def months_between(t1, t2)
+    months_excl_days = (t2.year*12 + t2.month) - (t1.year*12 + t1.month)
+    if t2.day > t1.day
+      months_excl_days
+    elsif t2.day < t1.day
+      months_excl_days - 1
+    elsif t2.seconds_since_midnight >= t1.seconds_since_midnight
+      months_excl_days
+    else
+      months_excl_days - 1
+    end
+  end
 
 end

spec/time_difference_spec.rb

@@ -181,4 +181,58 @@ def self.with_each_class(&block)
       end
     end
   end
+
+  describe '#in_calendar_months' do
+    with_each_class do |clazz|
+      it 'considers Sep-4 to Dec-4 to be 3 months' do
+        start_time = clazz.new(2017, 9, 4)
+        end_time = clazz.new(2017, 12, 4)
+
+        expect(TimeDifference.between(start_time, end_time).in_calendar_months).to eq(3)
+      end
+      it 'considers Aug-4 to Nov-4 to be 3 months' do
+        start_time = clazz.new(2017, 8, 4)
+        end_time = clazz.new(2017, 11, 4)
+
+        expect(TimeDifference.between(start_time, end_time).in_calendar_months).to eq(3)
+      end
+      it 'considers Sep-4 to Dec-3 to be 2 months' do
+        start_time = clazz.new(2017, 9, 4)
+        end_time = clazz.new(2017, 12, 3)
+
+        expect(TimeDifference.between(start_time, end_time).in_calendar_months).to eq(2)
+      end
+      it 'considers Sep-4 to Dec-31 to be 3 months' do
+        start_time = clazz.new(2017, 9, 4)
+        end_time = clazz.new(2017, 12, 31)
+
+        expect(TimeDifference.between(start_time, end_time).in_calendar_months).to eq(3)
+      end
+      it 'considers 2016-Sep-4 to 2017-Sep-4 to be 12 months' do
+        start_time = clazz.new(2016, 9, 4)
+        end_time = clazz.new(2017, 9, 4)
+
+        expect(TimeDifference.between(start_time, end_time).in_calendar_months).to eq(12)
+      end
+      it 'considers 2016-Sep-4 to 2017-Dec-31 to be 15 months' do
+        start_time = clazz.new(2016, 9, 4)
+        end_time = clazz.new(2017, 12, 31)
+
+        expect(TimeDifference.between(start_time, end_time).in_calendar_months).to eq(15)
+      end
+    end
+    it 'counts months on the day when time2 > time1' do
+      start_time = Time.new(2017, 9, 4, 12, 15)
+      end_time = Time.new(2017, 12, 4, 12, 22)
+
+      expect(TimeDifference.between(start_time, end_time).in_calendar_months).to eq(3)
+    end
+    it 'does not count months on the day when time2 < time1' do
+      start_time = Time.new(2017, 9, 4, 12, 15)
+      end_time = Time.new(2017, 12, 4, 12, 10)
+
+      expect(TimeDifference.between(start_time, end_time).in_calendar_months).to eq(2)
+    end
+  end
+
 end