This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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.