The input type="date-time" tag has been discontinued:
link
And was replaced by the input type="datetime-local" tag:
link
document.getElementById("datetime").defaultValue = new Date().toLocaleDateString();
<html>
<body>
<input id="datetime" type="datetime-local">
</body>
</html>
Note: Does not work in IE and Firefox: Chrome OK; Opera OK;
The type input type="date" is supported in firefox from version 57.0, in Chrome since 22;
document.getElementById("datetime").value = new Date().toLocaleDateString();
<html>
<body>
<input id="datetime" type="date">
</body>
</html>
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Considerations: There are still many inconsistencies in these inputs.
type="datetime-local" DOM properties defaultValue works only in firefox, chromium does not work but DOM properties
value works.
The type="date" input of the DOM porperties value does not work on
firefox also does not work on Chromium.
So, did your browser work?