Just upgraded from ww 2.15 to ww 2.16. The date-time-picker seems to have disappeared.
I have $options{useDateTimePicker} = 1; in default.config and it is also set in the course configuration. I cannot see any other place where it would be un-set.
There were some significant changes to the javascript in 2.16, so there are two things to check:
The date/time picker code has not changed from previous versions of webwork. Furthermore, there is no javascript usage that should be a problem for any modern browser. So everything should work fine on Vivaldi and Opera.