How to convert the file last modified timestamp to a date?

Date d = new Date(file.lastModified());

lastModified() returns the milliseconds since 1970-01-01, and the Date class stores its time also in the same way. The Date(long) constructor takes these milliseconds, and initializes the Date with it.

Just you use the SimpleDateFormat class to convert long to date. Only you execute code:

new SimpleDateFormat("dd-MM-yyyy HH-mm-ss").format(
    new Date(new File(filename).lastModified()) 
);

What you get is a long number representing the number of millis elapsed from Jan 1st, 1970. That's the standard way of representing dates.

try this:

java.util.Date myDate = new java.util.Date(theFile.lastModified());

and now you have a Date object at hand.

You can use SimpleDateFormat to print that date in a cuter way.