diff --git a/dbms/tests/queries/0_stateless/00935_to_iso_week_first_year.reference b/dbms/tests/queries/0_stateless/00935_to_iso_week_first_year.reference
new file mode 100644
index 00000000000..9e498be5588
--- /dev/null
+++ b/dbms/tests/queries/0_stateless/00935_to_iso_week_first_year.reference
@@ -0,0 +1,15 @@
+0000-00-00	1	1970
+1970-01-02	1	1970
+1970-01-03	1	1970
+1970-01-04	1	1970
+1970-01-05	2	1970
+1970-01-06	2	1970
+1970-01-07	2	1970
+1970-01-08	2	1970
+1970-01-09	2	1970
+1970-01-10	2	1970
+1970-01-11	2	1970
+1970-01-12	3	1970
+1970-01-13	3	1970
+1970-01-14	3	1970
+1970-01-15	3	1970
diff --git a/dbms/tests/queries/0_stateless/00935_to_iso_week_first_year.sql b/dbms/tests/queries/0_stateless/00935_to_iso_week_first_year.sql
new file mode 100644
index 00000000000..1691606a676
--- /dev/null
+++ b/dbms/tests/queries/0_stateless/00935_to_iso_week_first_year.sql
@@ -0,0 +1 @@
+SELECT toDate('1970-01-01') + number AS d, toISOWeek(d), toISOYear(d) FROM numbers(15);
\ No newline at end of file
diff --git a/libs/libcommon/include/common/DateLUTImpl.h b/libs/libcommon/include/common/DateLUTImpl.h
index 0a713207f60..28d536fc93d 100644
--- a/libs/libcommon/include/common/DateLUTImpl.h
+++ b/libs/libcommon/include/common/DateLUTImpl.h
@@ -371,7 +371,7 @@ public:
     /// The week number 1 is the first week in year that contains 4 or more days (that's more than half).
     inline unsigned toISOWeek(DayNum d) const
     {
-        return 1 + (toFirstDayNumOfWeek(d) - toFirstDayNumOfISOYear(d)) / 7;
+        return 1 + DayNum(toFirstDayNumOfWeek(d) - toFirstDayNumOfISOYear(d)) / 7;
     }
 
     inline unsigned toISOWeek(time_t t) const