diff --git a/js/src/plugins/dateFns.ts b/js/src/plugins/dateFns.ts
index 545428f0..0a88a9e3 100644
--- a/js/src/plugins/dateFns.ts
+++ b/js/src/plugins/dateFns.ts
@@ -5,10 +5,9 @@ export const dateFnsPlugin = {
   install(app: App, options: { locale: string }) {
     function dateFnsfileForLanguage(lang: string) {
       const matches: Record<string, string> = {
-        en_US: "en-US",
         en: "en-US",
       };
-      return matches[lang] ?? lang;
+      return matches[lang] ?? lang.replace("_", "-");
     }
 
     import(
diff --git a/js/src/views/Event/EditView.vue b/js/src/views/Event/EditView.vue
index ddb78ce9..d917583f 100644
--- a/js/src/views/Event/EditView.vue
+++ b/js/src/views/Event/EditView.vue
@@ -70,7 +70,7 @@
           class="datepicker starts-on"
           :placeholder="t('Type or select a date…')"
           icon="calendar-today"
-          :locale="$i18n.locale"
+          :locale="$i18n.locale.replace('_', '-')"
           v-model="beginsOn"
           horizontal-time-picker
           editable
@@ -90,7 +90,7 @@
           class="datepicker ends-on"
           :placeholder="t('Type or select a date…')"
           icon="calendar-today"
-          :locale="$i18n.locale"
+          :locale="$i18n.locale.replace('_', '-')"
           v-model="endsOn"
           horizontal-time-picker
           :min-datetime="beginsOn"