Changing "Event Duration" into an "End Time"

For non-recurring events, you define a start date and time and end date and time, but if you make it a recurring event, instead of being able to define the end time, you have to define the duration, which means calculating a possibly inaccurate fraction, e.g. 0.67 hours, which simply isn't practical.

How can we change this function to be an end time?