Timezone not working ok for the last of this month

[Valentina R]

Go here to reproduce.

The site is in some Australian timezone. It has slots on the 01.06.2020. Now change the timezone to Los Anegeles (USA). There should be available slots on the 31.05.2020. There are some and they are wrong and all of them show ‘Sorry, the selected slot is not available.’ when selected.

Reported here.