How can I let MATLAB add a row with a time format (00:00:00) to a csv file after selecting it

hi everyone! Hope you can help me with this issue. I want to create an app design that can analyse data acquired from an accelerometer. This data is imported to MATLAB from a csv file. I need this app to be able to add a row with the time into the selected data file so that MATLAB can see when a person is active from (for example) 09;00;15 - 10;30;00. The start time of the row is already given in a cell so is it possible to just select that cell and let MATLAB add a certain amount of seconds to this?

Thanks!