use vueuse

src/components/Timer.vue

@@ -2,7 +2,8 @@
     <span>{{ text }}</span>
 </template>
 <script setup>
-    import { computed, onBeforeUnmount, onMounted, ref } from 'vue';
+    import { useNow } from '@vueuse/core';
+    import { computed } from 'vue';
 
     import { timeToText } from '../shared/utils';
 
@@ -13,18 +14,8 @@
         }
     });
 
-    const now = ref(Date.now());
+    const now = useNow({ interval: 15000 });
     const text = computed(() => {
         return props.epoch ? timeToText(now.value - props.epoch) : '-';
     });
-
-    let timerId = null;
-    onMounted(() => {
-        timerId = setInterval(() => {
-            now.value = Date.now();
-        }, 15000);
-    });
-    onBeforeUnmount(() => {
-        clearInterval(timerId);
-    });
 </script>