整理した

src/client/app/common/views/components/timer.vue

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+<template>
+<time class="mk-time">
+	{{ hh }}:{{ mm }}:{{ ss }}
+</time>
+</template>
+
+<script lang="ts">
+import Vue from 'vue';
+
+export default Vue.extend({
+	props: {
+		time: {
+			type: [Date, String],
+			required: true
+		}
+	},
+	data() {
+		return {
+			tickId: null,
+			hh: null,
+			mm: null,
+			ss: null
+		};
+	},
+	computed: {
+		_time(): Date {
+			return typeof this.time == 'string' ? new Date(this.time) : this.time;
+		}
+	},
+	created() {
+		this.tick();
+		this.tickId = setInterval(this.tick, 1000);
+	},
+	destroyed() {
+		clearInterval(this.tickId);
+	},
+	methods: {
+		tick() {
+			const now = new Date().getTime();
+			const start = this._time.getTime();
+			const ago = Math.floor((now - start) / 1000);
+
+			this.hh = Math.floor(ago / (60 * 60)).toString().padStart(2, '0');
+			this.mm = Math.floor(ago / 60).toString().padStart(2, '0');
+			this.ss = (ago % 60).toString().padStart(2, '0');
+		}
+	}
+});
+</script>