JSFiddle - React, Tailwind, and code Playground

HTML

<html>
<head>
  <script src="https://unpkg.com/[email protected]/dayjs.min.js"></script>
  <script src="https://unpkg.com/[email protected]/plugin/customParseFormat.js"></script>
  <script>dayjs.extend(window.dayjs_plugin_customParseFormat)</script>
</head>

<body>
<script>
  let pat1 = 'YYYY-MM-DDTHH:mm:ss.SSS';
  let pat2 = 'YYYY-MM-DDTHH:mm:ss.SS';
  let pat3 = 'YYYY-MM-DDTHH:mm:ss.S';
  let pat4 = 'YYYY-MM-DDTHH:mm:ss';
  let pat5 = 'YYYY-MM-DD';

  console.log('Non-strict is OK');
  console.log(dayjs('2020-01-02T12:11:10.456', pat1).format(pat1));
  console.log(dayjs('2020-01-02T12:11:10.45', pat2).format(pat1));
  console.log(dayjs('2020-01-02T12:11:10.4', pat3).format(pat1));
  console.log(dayjs('2020-01-02T12:11:10.45', [pat1, pat2, pat3, pat4, pat5]).format(pat1));

  console.log('Strict is NOK');
  console.log(dayjs('2020-01-02T12:11:10.456', pat1, true).format(pat1));
  console.log(dayjs('2020-01-02T12:11:10.45', pat2, true).format(pat1));
  console.log(dayjs('2020-01-02T12:11:10.4', pat3, true).format(pat1));
  console.log(dayjs('2020-01-02T12:11:10.45', [pat1, pat2, pat3, pat4, pat5], true).format(pat1));
</script>
</body>

</html>