From 7fb7b51f1b36d49b576a359ee178d85e6d4b192a Mon Sep 17 00:00:00 2001
From: Geoff Lankow <geoff@darktrojan.net>
Date: Thu, 8 Sep 2022 15:44:51 +1200
Subject: [PATCH] Fix recurrence iteration where there is a negative BYMONTHDAY
 rule. (#530)

---
 lib/ical/recur_iterator.js  | 16 ++++++-----
 test/recur_iterator_test.js | 55 +++++++++++++++++++++++++++----------
 2 files changed, 50 insertions(+), 21 deletions(-)

diff --git a/lib/ical/recur_iterator.js b/lib/ical/recur_iterator.js
index 3a4570b1..c9d99f5b 100644
--- a/lib/ical/recur_iterator.js
+++ b/lib/ical/recur_iterator.js
@@ -214,7 +214,7 @@ ICAL.RecurIterator = (function() {
       this.last.second = this.setup_defaults("BYSECOND", "SECONDLY", this.dtstart.second);
       this.last.minute = this.setup_defaults("BYMINUTE", "MINUTELY", this.dtstart.minute);
       this.last.hour = this.setup_defaults("BYHOUR", "HOURLY", this.dtstart.hour);
-      this.last.day = this.setup_defaults("BYMONTHDAY", "DAILY", this.dtstart.day);
+      var dayOffset = this.last.day = this.setup_defaults("BYMONTHDAY", "DAILY", this.dtstart.day);
       this.last.month = this.setup_defaults("BYMONTH", "MONTHLY", this.dtstart.month);
 
       if (this.rule.freq == "WEEKLY") {
@@ -301,13 +301,15 @@ ICAL.RecurIterator = (function() {
           throw new Error("Malformed values in BYDAY part");
         }
 
-      } else if (this.has_by_data("BYMONTHDAY")) {
-        if (this.last.day < 0) {
-          var daysInMonth = ICAL.Time.daysInMonth(this.last.month, this.last.year);
-          this.last.day = daysInMonth + this.last.day + 1;
-        }
-      }
+      } else if (this.has_by_data("BYMONTHDAY") && dayOffset < 0) {
+        // Attempting to access `this.last.day` will cause the date to be normalised and
+        // not return a negative value. We keep the value in a separate variable instead.
 
+        // Now change the day value so that normalisation won't change the month.
+        this.last.day = 1;
+        var daysInMonth = ICAL.Time.daysInMonth(this.last.month, this.last.year);
+        this.last.day = daysInMonth + dayOffset + 1;
+      }
     },
 
     /**
diff --git a/test/recur_iterator_test.js b/test/recur_iterator_test.js
index 2eaf4737..99456fbf 100644
--- a/test/recur_iterator_test.js
+++ b/test/recur_iterator_test.js
@@ -516,12 +516,12 @@ suite('recur_iterator', function() {
 
       // monthly, the third instance of tu,we,th
       testRRULE('FREQ=MONTHLY;COUNT=3;BYDAY=TU,WE,TH;BYSETPOS=3', {
-          byCount: true,
-          dates: [
-            '1997-09-04T09:00:00',
-            '1997-10-07T09:00:00',
-            '1997-11-06T09:00:00'
-          ]
+        byCount: true,
+        dates: [
+          '1997-09-04T09:00:00',
+          '1997-10-07T09:00:00',
+          '1997-11-06T09:00:00'
+        ]
       });
 
       //monthly, each month last day that is monday
@@ -604,7 +604,8 @@ suite('recur_iterator', function() {
           '2015-01-19T08:00:00',
           '2015-01-21T08:00:00',
           '2015-01-23T08:00:00'
-        ]});
+        ]
+      });
 
       //Repeat Monthly, the fifth Saturday (BYDAY=5SA)
       testRRULE('FREQ=MONTHLY;BYDAY=5SA', {
@@ -616,8 +617,8 @@ suite('recur_iterator', function() {
           '2016-01-30T08:00:00',
           '2016-04-30T08:00:00',
           '2016-07-30T08:00:00'
-       ]
-     });
+        ]
+      });
 
       // Repeat Monthly, the fifth Wednesday every two months (BYDAY=5WE)
       testRRULE('FREQ=MONTHLY;INTERVAL=2;BYDAY=5WE', {
@@ -666,7 +667,7 @@ suite('recur_iterator', function() {
         ]
       });
 
-      // monthly, bymonthday
+      // Last day of the month, monthly.
       testRRULE('FREQ=MONTHLY;BYMONTHDAY=-1', {
         dtStart: '2015-01-01T08:00:00',
         dates: [
@@ -676,6 +677,32 @@ suite('recur_iterator', function() {
         ]
       });
 
+      // Last day of the month, every 3 months.
+      testRRULE('FREQ=MONTHLY;INTERVAL=3;BYMONTHDAY=-1', {
+        dtStart: '2022-06-01T08:00:00',
+        dates: [
+          '2022-06-30T08:00:00',
+          '2022-09-30T08:00:00',
+          '2022-12-31T08:00:00',
+          '2023-03-31T08:00:00',
+          '2023-06-30T08:00:00',
+          '2023-09-30T08:00:00',
+        ]
+      });
+
+      // Second-to-last day of the month, every 3 months.
+      testRRULE('FREQ=MONTHLY;INTERVAL=3;BYMONTHDAY=-2', {
+        dtStart: '2022-06-01T08:00:00',
+        dates: [
+          '2022-06-29T08:00:00',
+          '2022-09-29T08:00:00',
+          '2022-12-30T08:00:00',
+          '2023-03-30T08:00:00',
+          '2023-06-29T08:00:00',
+          '2023-09-29T08:00:00',
+        ]
+      });
+
       // monthly + by month
       testRRULE('FREQ=MONTHLY;BYMONTH=1,3,6,9,12', {
         dates: [
@@ -696,7 +723,7 @@ suite('recur_iterator', function() {
           '2015-03-23T08:00:00Z',
           '2015-03-27T08:00:00Z',
         ]
-      })
+      });
       testRRULE('FREQ=MONTHLY;BYDAY=MO,FR;BYMONTHDAY=1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31;COUNT=4', {
         dtStart: '2015-04-01T08:00:00Z',
         byCount: true,
@@ -706,7 +733,7 @@ suite('recur_iterator', function() {
           '2015-04-17T08:00:00Z',
           '2015-04-27T08:00:00Z'
         ]
-      })
+      });
       testRRULE('FREQ=MONTHLY;BYDAY=MO,SA;BYMONTHDAY=1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31;COUNT=4', {
         dtStart: '2015-04-01T08:00:00Z',
         byCount: true,
@@ -716,7 +743,7 @@ suite('recur_iterator', function() {
           '2015-04-25T08:00:00Z',
           '2015-04-27T08:00:00Z'
         ]
-      })
+      });
       testRRULE('FREQ=MONTHLY;BYDAY=SU,FR;BYMONTHDAY=1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31;COUNT=9', {
         dtStart: '2015-02-28T08:00:00Z',
         byCount: true,
@@ -731,7 +758,7 @@ suite('recur_iterator', function() {
           "2015-04-17T08:00:00Z",
           "2015-04-19T08:00:00Z"
         ]
-      })
+      });
     });
 
     suite('YEARLY', function() {