Использовать календарь:
Calendar cal = Calendar.getInstance();
int yearX = 1980;
cal.set(Calendar.YEAR, yearX);
int monthX = 5;
cal.set(Calendar.MONTH, monthX);
int dayX = 15;
cal.set(Calendar.DAY_OF_MONTH, dayX);
int hourX = 1;
cal.set(Calendar.HOUR_OF_DAY, hourX);
int minX = 25;
cal.set(Calendar.MINUTE, minX);
int secX = 55;
cal.set(Calendar.SECOND, secX);
int milX = 15;
cal.set(Calendar.MILLISECOND, milX);
long millis = cal.getTimeInMillis();