diff --git a/src/locale.js b/src/locale.js
index <HASH>..<HASH> 100644
--- a/src/locale.js
+++ b/src/locale.js
@@ -621,7 +621,7 @@ function formatUTCWeekdayNumberMonday(d) {
 }
 
 function formatUTCWeekNumberSunday(d, p) {
-  return pad(utcSunday.count(utcYear(d), d), p, 2);
+  return pad(utcSunday.count(utcYear(d) - 1, d), p, 2);
 }
 
 function formatUTCWeekNumberISO(d, p) {
@@ -635,7 +635,7 @@ function formatUTCWeekdayNumberSunday(d) {
 }
 
 function formatUTCWeekNumberMonday(d, p) {
-  return pad(utcMonday.count(utcYear(d), d), p, 2);
+  return pad(utcMonday.count(utcYear(d) - 1, d), p, 2);
 }
 
 function formatUTCYear(d, p) {
Fix off-by-one in utcFormat.