Java time since the epoch

In Java, how can I print out the time since the epoch given in seconds and nanoseconds in the following format :

java.text.SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");

My input is:

long mnSeconds;
long mnNanoseconds;

Where the total of the two is the elapsed time since the epoch 1970-01-01 00:00:00.0.