Set start time for arduino uno clock
Assume you want the clock to start at 08:42:21, change

int seconds, minutes, hours;


to 

int seconds = 21, minutes = 42, hours = 8;


Note that you should always also initialize (as in the second case above), and not only declare (as in the first case) all variables before you first read their values. Otherwise, for local variables you cannot be sure what their initial value is. (Global variables are implicitly initialized to zero if not explicitly initialized.)