@lauri What browser & os are your clients using? Different browsers and OS report dates differently. Some browsers use the system clock, some don’t. Day.js might be reacting to that i guess. I had a hell of a time once trying to get a date picker to show UK style dates in IE which was showing US style dates, but all the other browsers were correctly showing UK dates (or vice versa). As I recall, IE gets the date and time from the OS, other browsers don’t. Something like that.
trouble is, I think it can only fixed in Kirby itself, to force the date formatting on the picker. I do recall their being a browser setting in IE that you can change how it reports dates. You could in the meantime get your US clients to investigate that.