Insert a date-picker image
If you're talking about the back end, Deborah is right. All you'd have to do is check the Use Datepicker Display datepicker icon and popup calendar beside date fields checkbox on the Admin>General page.
If you're talking about a form, here's a thread that might help: https://www.interactivetools.com/forum/forum-posts.php?postNum=2230131
Otherwise, doing a Google search for something like datepicker in html form comes up with many options.
Hope that helps
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