These two things
"only get the data for the day at 00:00. I want you to take all the data from those selected days"
It sounds like you have a timetable that spans multiple days, with 15min time steps. If you want to select only some of those days, and they are contiguous, use timerange as Steve suggests. If the days are not contiguous, use ismember to create a logical vector from the timetable's row times, then use that as a row subscript to select the days of interest.
If you additionally only want the "midnight" rows on those selected days, use timeofday on the timetable's row times to again create a logical vector for subscripting.
If, on the other hand, you want to somehow aggregate all the data from each day into one row whose timestamp is at midnight, use retime with an aggregation, as KSSV suggests.
It would help if you gavce a short clear example of what you have and what you want.