Sum certain rows for a specific column in a table

I am starting in matlab and I have a table of 4 columns and x rows (didn't count them).

It is a table where the first column is the time (example : January 1st 2017 00:00). The 3 others are data numbers

I am searching to add every n rows the data of a specific row (the 3rd and the 4th) and I would like to return these values to a table where I have the day (for example January 1st) and the cell next to it (same row) is the addition of the n rows.

I would like to do so for every day of the year.

I know it is possible on Excel by doing a sum and selecting the specific row but it taks forever to do so I was hoping matlab had a faster way...

[per isakson]

See timetable, Create timetable from workspace variables

[KL]

For older versions, use varfun with tables.