Date picker overprints itself

https://wafertest.debconf.org/register/step-8 says

Arrival July 23, 2018, midnight estimated

OK I went back to fix it on https://wafertest.debconf.org/register/step-2

and cannot click any earlier days with the mouse, as you see, the date picker overprints itself:

The only way to make it the 21st is to change 2018-07-23 into 2018-07-21 by editing the date text string in the textbox.