How can an admin set the datetime format?

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2

You can change the client-side ones with code in </head>:

<script>
I18n.translations.en.js.dates.time = "HH:mm"
I18n.translations.en.js.dates.long_with_year = "DD MMM YYYY HH:mm:ss"
I18n.translations.en.js.dates.long_date_with_year = "DD MMM YYYY HH:mm:ss"
I18n.translations.en.js.dates.long_no_year_no_time = "DD MMM YYYY"
I18n.translations.en.js.dates.long_with_year_no_time = "DD MMM YYYY"
</script>

Build the rest using Moment.js | Docs