This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I guess the seconds are the number of seconds since a predefined start time (for example January 1, 1970, 00:00 GMT)?
Create a java.util.Date object, set the time on the object using your number of seconds (you'll have to multiply it with 1000 to get milliseconds), and use java.text.SimpleDateFormat to format it to a string in the desired format. See the API documentation.