If all the entries in the Time variable in your table were valid date strings that datetime could understand, I would first convert that variable into a datetime array. Next I would call table2timetable to make a timetable from the table. Finally I would call retime or groupsummary on that timetable.
As shown the Time variable in the table contains at least one element that datetime will not be able to understand: 'Start'. Can you remove that or replace it with the actual starting time for your data set? If so you could try the process I described.
