You are gonna need a variable that keeps track on the time that has passed since the Chronometer was started:
long timeWhenStopped = 0;
Update the value of the variable when you stop the chronometer like this:
timeWhenStopped = mChronometer.getBase() - SystemClock.elapsedRealtime();
mChronometer.stop();
We will also use this variable to adjust the chronometer before starting it:
mChronometer.setBase(SystemClock.elapsedRealtime() + timeWhenStopped);
mChronometer.start();
And finally if you have a way to reset your chronometer then you should remember to also reset the timeWhenStopped variable. Something like this:
mChronometer.setBase(SystemClock.elapsedRealtime());
timeWhenStopped = 0;