@lostfrogg If you set Runtime property as "Current Date/Time", it calls the below FormCalc script in layout:ready event of the DatatimeField. If you want to change the display pattern, you can modify it here.
$.rawValue = Concat(Num2Date(Date(), "YYYY-MM-DD"), " ", Num2Time(Time(), "HH:MM:SS"))
Also, if you want to show the date & time when the user opens the form to fill it, in that case, you can write a simple Javascript for fetching the current datetime and use it in the form initialize event.
I hope this is helpful.