diff --git a/src/weekUtils.ts b/src/weekUtils.ts
index d8ac22b..195ab40 100644
--- a/src/weekUtils.ts
+++ b/src/weekUtils.ts
@@ -5,11 +5,10 @@ export const getWeekNumber = (date: Date) => {
 
 export const getWeekOfYear = (): number => getWeekNumber(new Date());
 
-export const getWeekIsoNumber = (date: Date) => {
-    const onejan = new Date(date.getFullYear(), 0, 1);
-    const daysSinceOneJan = Math.floor((date.getTime() - onejan.getTime()) / 86400000);
-    const dayOfWeek = (onejan.getDay() + 6) % 7;
-    return Math.floor((daysSinceOneJan + dayOfWeek) / 7) + 1;
+export const getWeekIsoNumber = (date: Date): number => {
+    const dayOfWeek = date.getDay();
+    const thursday = new Date(date.getFullYear(), date.getMonth(), date.getDate() + (4 - dayOfWeek));
+    return Math.ceil(((thursday.getTime() - new Date(thursday.getFullYear(), 0, 1).getTime()) / 86400000 + 1) / 7);
 };
 
 export const getWeekIsoOfYear = (): number => getWeekIsoNumber(new Date());