Update needed for Multisite

In a multisite the selected date format is German: dd.mm.yyyy. The date picker still uses mm.dd.yyyy.
When saving a date with day > 12 (interpreted as month), the month is invalid, so date is reset to 01.01.1970.
When a day <= 12 is selected, day and month are switched, and with every saving, they change position each time.
This happens only in a multi-site, a single site works as expected.

https://github.com/wpeventmanager/wp-event-manager/issues/809#issuecomment-1049637956