Fix year-over-year comparisons for leap years (#4940)

When comparing dates year-over-year, the previous logic always shifted
by 365 days, which caused issues with leap years.

This means when comparing e.g. last 7 days vs a year ago in 2025, you
would compare Jan 6th and Jan 7th.

The new logic uses `Date.shift` while ensuring that the compared date
range stays the same length as original date range. This can cause some
oddities around Feb 29th where dates can get offset by 1 at the end of
the range (see tests)

Helpscout ref: https://secure.helpscout.net/conversation/2809180400/22224?viewId=6980900

Author: macobo

CHANGELOG.md

@@ -23,6 +23,7 @@ All notable changes to this project will be documented in this file.
 - Fix returning filter suggestions for multiple custom property values in the dashboard Filter modal
 - Fix typo on login screen
 - Fix Direct / None details modal not opening
+- Fix year over year comparisons being offset by a day for leap years
 
 ## v2.1.4 - 2024-10-08
 

lib/plausible/stats/comparisons.ex

@@ -99,8 +99,9 @@ defmodule Plausible.Stats.Comparisons do
   defp get_comparison_date_range(source_query, %{mode: "year_over_year"} = options) do
     source_date_range = Query.date_range(source_query, trim_trailing: true)
 
-    start_date = Date.add(source_date_range.first, -365)
-    end_date = source_date_range.last |> Date.add(-365)
+    start_date = source_date_range.first |> Date.shift(year: -1)
+    diff_in_days = Date.diff(source_date_range.last, source_date_range.first)
+    end_date = Date.add(start_date, diff_in_days)
 
     Date.range(start_date, end_date)
     |> maybe_match_day_of_week(source_date_range, options)

test/plausible/stats/comparisons_test.exs

@@ -161,6 +161,36 @@ defmodule Plausible.Stats.ComparisonsTest do
     end
   end
 
+  describe "year_over_year, exact dates behavior with leap years" do
+    test "start of the year matching", %{site: site} do
+      query = Query.from(site, %{"period" => "7d", "date" => "2021-01-05"})
+      comparison_query = Comparisons.get_comparison_query(query, %{mode: "year_over_year"})
+
+      assert comparison_query.utc_time_range.first == ~U[2019-12-30 00:00:00Z]
+      assert comparison_query.utc_time_range.last == ~U[2020-01-05 23:59:59Z]
+      assert date_range_length(comparison_query) == 7
+    end
+
+    test "leap day matching", %{site: site} do
+      query = Query.from(site, %{"period" => "7d", "date" => "2021-03-03"})
+      comparison_query = Comparisons.get_comparison_query(query, %{mode: "year_over_year"})
+
+      assert comparison_query.utc_time_range.first == ~U[2020-02-25 00:00:00Z]
+      # :TRICKY: Since dates of the two months don't match precisely we cut off earlier
+      assert comparison_query.utc_time_range.last == ~U[2020-03-02 23:59:59Z]
+      assert date_range_length(comparison_query) == 7
+    end
+
+    test "end of the year matching", %{site: site} do
+      query = Query.from(site, %{"period" => "7d", "date" => "2021-11-25"})
+      comparison_query = Comparisons.get_comparison_query(query, %{mode: "year_over_year"})
+
+      assert comparison_query.utc_time_range.first == ~U[2020-11-19 00:00:00Z]
+      assert comparison_query.utc_time_range.last == ~U[2020-11-25 23:59:59Z]
+      assert date_range_length(comparison_query) == 7
+    end
+  end
+
   describe "with period set to year to date" do
     test "shifts back by the same number of days when mode is previous_period", %{site: site} do
       query =
@@ -369,4 +399,10 @@ defmodule Plausible.Stats.ComparisonsTest do
 
     query
   end
+
+  def date_range_length(query) do
+    query
+    |> Query.date_range()
+    |> Enum.count()
+  end
 end