diff --git a/lib/icinga/legacytimeperiod.cpp b/lib/icinga/legacytimeperiod.cpp
index 33e666544..212ff8439 100644
--- a/lib/icinga/legacytimeperiod.cpp
+++ b/lib/icinga/legacytimeperiod.cpp
@@ -62,18 +62,21 @@ void LegacyTimePeriod::FindNthWeekday(int wday, int n, tm *reference)
 
 	if (n > 0) {
 		dir = 1;
+
+		/* Postitive days are relative to the first day of the month. */
+		t.tm_mday = 1;
 	} else {
 		n *= -1;
 		dir = -1;
 
-		/* Negative days are relative to the next month. */
+		/* Negative days are relative to the last day of the month which is
+		 * what mktime() normalizes the 0th day of the next month to. */
 		t.tm_mon++;
+		t.tm_mday = 0;
 	}
 
 	ASSERT(n > 0);
 
-	t.tm_mday = 1;
-
 	for (;;) {
 		// Always operate on 00:00:00 with automatic DST detection, otherwise days could
 		// be skipped or counted twice if +-24 hours is not on the next or previous day.
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index d2ce3a0ce..defc5e889 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -188,6 +188,7 @@ add_boost_test(base
     icinga_legacytimeperiod/advanced
     icinga_legacytimeperiod/dst
     icinga_legacytimeperiod/dst_isinside
+    icinga_legacytimeperiod/find_nth_weekday
     icinga_perfdata/empty
     icinga_perfdata/simple
     icinga_perfdata/quotes
diff --git a/test/icinga-legacytimeperiod.cpp b/test/icinga-legacytimeperiod.cpp
index e1150be57..50cad3f28 100644
--- a/test/icinga-legacytimeperiod.cpp
+++ b/test/icinga-legacytimeperiod.cpp
@@ -691,4 +691,56 @@ BOOST_AUTO_TEST_CASE(dst_isinside)
 	}
 }
 
+BOOST_AUTO_TEST_CASE(find_nth_weekday) {
+	auto run = [](const std::string& refDay, int wday, int n, const std::string& expectedDay) {
+		tm expected = make_tm(expectedDay + " 00:00:00");
+
+		tm t = make_tm(refDay + " 00:00:00");
+		LegacyTimePeriod::FindNthWeekday(wday, n, &t);
+
+		BOOST_CHECK_MESSAGE(mktime(&expected) == mktime(&t),
+			"[ref=" << refDay << ", wday=" << wday << ", n=" << n << "] "
+			"expected: " << pretty_time(expected) << ", "
+			"got: " << pretty_time(t));
+	};
+
+	/*      March 2019
+	 * Mo Tu We Th Fr Sa Su
+	 *              1  2  3
+	 *  4  5  6  7  8  9 10
+	 * 11 12 13 14 15 16 17
+	 * 18 19 20 21 22 23 24
+	 * 25 26 27 28 29 30 31
+	 */
+
+	// Use every day of the month as reference day, all must give the same result for that month.
+	for (int i = 1; i <= 31; ++i) {
+		std::stringstream refDayStream;
+		refDayStream << "2019-03-" << std::setw(2) << std::setfill('0') << i;
+		std::string refDay = refDayStream.str();
+
+		const int monday = 1; // 4 ocurrences in March 2019
+		run(refDay, monday, 1, "2019-03-04");
+		run(refDay, monday, 2, "2019-03-11");
+		run(refDay, monday, 3, "2019-03-18");
+		run(refDay, monday, 4, "2019-03-25");
+		run(refDay, monday, -1, "2019-03-25");
+		run(refDay, monday, -2, "2019-03-18");
+		run(refDay, monday, -3, "2019-03-11");
+		run(refDay, monday, -4, "2019-03-04");
+
+		const int friday = 5; // 5 ocurrences in March 2019
+		run(refDay, friday, 1, "2019-03-01");
+		run(refDay, friday, 2, "2019-03-08");
+		run(refDay, friday, 3, "2019-03-15");
+		run(refDay, friday, 4, "2019-03-22");
+		run(refDay, friday, 5, "2019-03-29");
+		run(refDay, friday, -1, "2019-03-29");
+		run(refDay, friday, -2, "2019-03-22");
+		run(refDay, friday, -3, "2019-03-15");
+		run(refDay, friday, -4, "2019-03-08");
+		run(refDay, friday, -5, "2019-03-01");
+	}
+}
+
 BOOST_AUTO_TEST_SUITE_END()