The culprit is the following function:
StackExchange.loadJqueryUi = function() {
var cssLink = StackExchange.settings.paths.jQueryUICSSPath
, jsLink = StackExchange.settings.paths.jQueryUIJSPath;
if ($.ui)
return $.Deferred().resolve();
/* Some AJAX loader code... */
}
...which was called by:
$(function() {
StackExchange.loadJqueryUi().done(showCalendar);
});
function showCalendar() {
var $cal = $('.js-daily-access-calendar');
$cal.datepicker({ /* Options... */ });
}
StackExchange.loadJqueryUi
assumes that $.ui
exists also means $.datepicker()
is available, which was not the case. I tried two crude monkeypatches; they both worked:
StackExchange._loadJqueryUi = StackExchange.loadJqueryUi
StackExchange.loadJqueryUi = function() {
delete $.ui;
return StackExchange._loadJqueryUi();
}
StackExchange.loadJqueryUi = function() {
var cssLink = StackExchange.settings.paths.jQueryUICSSPath
, jsLink = StackExchange.settings.paths.jQueryUIJSPath;
if ($.datepicker)
return $.Deferred().resolve();
/* The rest... */
}
TLDR: Run delete $.ui
in your browser console.
Whether they break anything and how jQueryUI was loaded without .datepicker()
in the first place are unknown. I didn't bother to check further, since all I want was to see my SO visit calendar. Isn't she beautiful?

delete $.ui
in the browser console prior to trying to open the calendar and have the calendar displayed normally.