Nicole, the following example should help you in what you are trying to do. I have added some comments in the code to help you. To perform arithmetic operations on dates and times, you must ensure that they have the same format as shown below.
It initially creates a table which looks like this:
dates times
__________ __________
09/29/2017 7.3706e+05
09/28/2017 7.3706e+05
09/27/2017 7.3706e+05
as you indicated that you have dates in the first column and datenums in the second column.
%First column of table with dates.
DateStrings = {'29/09/2017','28/09/2017','27/09/2017'};
dates = datetime(DateStrings,'Format','MM/dd/yyyy')';
%Second column of table with times in datenum format.
TimeStrings = {'20:30:30','19:30:21','14:32:01'};
times = datenum(TimeStrings);
%This will create the table as shown in the first part of the answer.
t = table(dates,times);
%Format both columns to MM/dd/yyyy HH:mm:SS for addition.
dates = datetime(t.dates,'Format','MM/dd/yyyy HH:mm:SS');
times = datetime(t.times,'ConvertFrom','datenum','Format','MM/dd/yyyy HH:mm:SS');
%Add dates to times.
fullt = dates+timeofday(times);
t.DatesNTimes = fullt;
The final table contains the third additional column which is the result of the addition of the date of the first and the time of the second.
dates times DatesNTimes
__________ __________ ___________________
09/29/2017 7.3706e+05 09/29/2017 20:30:00
09/28/2017 7.3706e+05 09/28/2017 19:30:00
09/27/2017 7.3706e+05 09/27/2017 14:32:00
