The html5 fields provided by WTForms do include a DateTimeLocalField - v2.3.x, v3.0.x. Thus (v2.3)

from wtforms.fields.html5 import DateTimeLocalField
dttm = DateTimeLocalField("Date/Time: ", format="%Y-%m-%dT%H:%M")

will show the html5 date picker and time picker popups on browsers that support them (support is varied).

I find you need to specify format="%Y-%m-%dT%H:%M" - note the T - because the default format="%Y-%m-%d %H:%M" fails validation (see quote below).

input type="datetime" appears to be deprecated, so browsers fall back to plain text entry: input type="datetime" | MDN. input type="datetime-local", and thus WTForms DateTimeLocalField, should always be used: input type="datetime-local" | MDN.

From the latter:

One thing to note is that the displayed date and time formats differ from the actual value; the displayed date and time are formatted according to the user's locale as reported by their operating system, whereas the date/time value is always formatted YYYY-MM-DDThh:mm.

Ok after being forced to read the WTForms documentation because nobody immediately answered my question I now understand that it can be manipulated to do anything HTML can do by passing in HTML keyword arguments like so:

{{ form.when.label }}<br>
{{ form.when(type='datetime-local') }}