Definitely affects trunk, most likely affects other versions too, but I have not checked.

To reproduce you need to set different time zones in php.ini file and system-wide, system time zone must be ahead of PHP date.timezone, for example:

$ cat /etc/php/7.0/apache2/php.ini | grep 'date\.timezone ='
date.timezone = Europe/Riga

$ cat /etc/timezone
Asia/Tokyo

Then go to Configuration->Maintenance and press Create maintenance period.

Press the little calendar button next to Active since or Active till settings.

You will see that day in the calendar is by one less than day in the input field:

If you press Done this incorrect day will propagate into input field:

P.S. Since web server and web browser are running on the same system in my case, I don't know which of them is affected by system settings. But these is clearly some JavaScript client - JavaScript server - PHP magic going on.
P.P.S. Strangely, this calendar selector is used in many places across frontend (e.g. in graphs and screens), but only maintenance form seems to be affected.