I also had this issue, and for anyone else that needs help, I solved it by casting the
from_dataset() function to a date using the
date(from_dataset("RESERVATION", "ResDateTime", ResID: __ResID)) + minutes(random(10,15))
Otherwise, you are attempting to add a duration value (a number of days) to a string. This converts it to a duration value which can be manipulated.
Hope this helps.