How to store the time data in a timer program

I have a program in Java Swing that has stop and start buttons that track the time that passed since the start button was pressed and display it via JLabel. What I want to do now is to store the time in some way. But I’m not sure how to go about this. Should I store it in a .txt, .ini or .xml file? Or use some other file type? The data to be stored includes time: pairs d:m:y and [h:m:s, h:m:s].