diff --git a/src/shared/calendarspec.c b/src/shared/calendarspec.c
index d83e0fbded3..48041bdfa10 100644
--- a/src/shared/calendarspec.c
+++ b/src/shared/calendarspec.c
@@ -1185,15 +1185,18 @@ static int tm_within_bounds(struct tm *tm, bool utc) {
                 return negative_errno();
 
         /* Did any normalization take place? If so, it was out of bounds before */
-        bool good = t.tm_year == tm->tm_year &&
-                    t.tm_mon  == tm->tm_mon  &&
-                    t.tm_mday == tm->tm_mday &&
-                    t.tm_hour == tm->tm_hour &&
-                    t.tm_min  == tm->tm_min  &&
-                    t.tm_sec  == tm->tm_sec;
-        if (!good)
+        int cmp = CMP(t.tm_year, tm->tm_year) ?:
+                  CMP(t.tm_mon, tm->tm_mon) ?:
+                  CMP(t.tm_mday, tm->tm_mday) ?:
+                  CMP(t.tm_hour, tm->tm_hour) ?:
+                  CMP(t.tm_min, tm->tm_min) ?:
+                  CMP(t.tm_sec, tm->tm_sec);
+
+        if (cmp < 0)
+                return -EDEADLK; /* Refuse to go backward */
+        if (cmp > 0)
                 *tm = t;
-        return good;
+        return cmp == 0;
 }
 
 static bool matches_weekday(int weekdays_bits, const struct tm *tm, bool utc) {
diff --git a/src/test/test-calendarspec.c b/src/test/test-calendarspec.c
index 9e2ae55ab60..080aa1f0124 100644
--- a/src/test/test-calendarspec.c
+++ b/src/test/test-calendarspec.c
@@ -218,6 +218,9 @@ int main(int argc, char* argv[]) {
         // Confirm that timezones in the Spec work regardless of current timezone
         test_next("2017-09-09 20:42:00 Pacific/Auckland", "", 12345, 1504946520000000);
         test_next("2017-09-09 20:42:00 Pacific/Auckland", "EET", 12345, 1504946520000000);
+        /* Check that we don't start looping if mktime() moves us backwards */
+        test_next("Sun *-*-* 01:00:00 Europe/Dublin", "", 1616412478000000, 1617494400000000);
+        test_next("Sun *-*-* 01:00:00 Europe/Dublin", "IST", 1616412478000000, 1617494400000000);
 
         assert_se(calendar_spec_from_string("test", &c) < 0);
         assert_se(calendar_spec_from_string(" utc", &c) < 0);
diff --git a/test/test-functions b/test/test-functions
index 984643c37fa..cc817925f0a 100644
--- a/test/test-functions
+++ b/test/test-functions
@@ -1121,6 +1121,7 @@ install_zoneinfo() {
     inst_any /usr/share/zoneinfo/Asia/Vladivostok
     inst_any /usr/share/zoneinfo/Australia/Sydney
     inst_any /usr/share/zoneinfo/Europe/Berlin
+    inst_any /usr/share/zoneinfo/Europe/Dublin
     inst_any /usr/share/zoneinfo/Europe/Kiev
     inst_any /usr/share/zoneinfo/Pacific/Auckland
     inst_any /usr/share/zoneinfo/Pacific/Honolulu