We have also seen this become an issue recently. It appears the place to edit the time has moved. Try typing in the actual text box where the date and time displays. This workaround has been working for us. If you try to use the time field under the calendar, it kicks us out also. Let me know if this works, please!
Thanks for the suggestion Paula! This did get fixed by WF but I'll keep your solution in mind in case a similar issue comes up. Thanks so much!!!