Fix division in Function.weekdiff() to avoid issue near start of epoch

h2/src/main/org/h2/expression/Function.java

@@ -1915,11 +1915,13 @@ public class Function extends Expression implements FunctionCall {
     }
 
     private static long weekdiff(long absolute1, long absolute2, int firstDayOfWeek) {
-        long r1 = (absolute1 - firstDayOfWeek + 4) / 7;
+        absolute1 += 4 - firstDayOfWeek;
+        long r1 = absolute1 / 7;
         if (absolute1 < 0 && (r1 * 7 != absolute1)) {
             r1--;
         }
-        long r2 = (absolute2 - firstDayOfWeek + 4) / 7;
+        absolute2 += 4 - firstDayOfWeek;
+        long r2 = absolute2 / 7;
         if (absolute2 < 0 && (r2 * 7 != absolute2)) {
             r2--;
         }

h2/src/test/org/h2/test/scripts/functions/timeanddate/datediff.sql

@@ -182,3 +182,15 @@ SELECT DATEDIFF('WEEK', DATE '2018-02-05', DATE '2018-02-06'), DATEDIFF('ISO_WEE
 > - -
 > 0 0
 > rows: 1
+
+SELECT DATEDIFF('WEEK', DATE '1969-12-27', DATE '1969-12-28'), DATEDIFF('ISO_WEEK', DATE '1969-12-27', DATE '1969-12-28');
+> 1 0
+> - -
+> 1 0
+> rows: 1
+
+SELECT DATEDIFF('WEEK', DATE '1969-12-28', DATE '1969-12-29'), DATEDIFF('ISO_WEEK', DATE '1969-12-28', DATE '1969-12-29');
+> 0 1
+> - -
+> 0 1
+> rows: 1