LWCでの共通Util自分で作る

/**
 * デートフォマート
 * @param {Date} date date
 * @param {string} fmt format
 * @returns {string} StringDate
 */
 export const dateFormat = (date, fmt = 'YYYY/mm/dd') => {
    let ret;
    const opt = {
        'Y+': date.getFullYear().toString(), // 年
        'm+': (date.getMonth() + 1).toString(), // 月
        'd+': date.getDate().toString(), // 日
        'H+': date.getHours().toString(), // 時
        'M+': date.getMinutes().toString(), // 分
        'S+': date.getSeconds().toString() // 秒
    };
    for (let k in opt) {
        ret = new RegExp('(' + k + ')').exec(fmt);
        if (ret) {
            fmt = fmt.replace(ret[1], (ret[1].length == 1) ? (opt[k]) : (opt[k].padStart(ret[1].length, '0')))
        };
    };
    return fmt;
}

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
    <apiVersion>52.0</apiVersion>
    <isExposed>false</isExposed>
</LightningComponentBundle>

<template>
    <div class="slds-card" style="height:500px;width:1200px">{dateStr}</div>
</template>

import { LightningElement, track } from 'lwc';
import { dateFormat } from 'c/commonUtil';
export default class CommonUtilChild extends LightningElement {
    @track dateStr;

    connectedCallback() {
        this.timer = setInterval(() => {
            this.dateStr = dateFormat(new Date(), 'YYYY/mm/dd HH:MM:SS');
        })
    }

    disconnectedCallback() {
        clearInterval(this.timer);
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
    <apiVersion>52.0</apiVersion>
    <isExposed>false</isExposed>
</LightningComponentBundle>