Resetting ofGetElapsedTimeMilles

hi i been trying to make an FBO to fade away in a short amount of time
but i like it to happen continuously thats why im trying to use ofGetElapsedTimeMilles so when it get to a second or more the fbo clears but i like it to happen all the time so is there a way to reset the ofGetElapsedTimeMilles value back to 0 and to started again so it happens continuously to the fbo can someone give a clue how to ?
on the ofApp.h
/////////////////////////////
float startTime;
float endTime;
bool bTimerReached;
/////////////////////////////
ofApp.cpp
setup()
startTime =ofGetElapsedTimeMillis();
endTime =1000;
bTimerReached =false;
update()
float timer = ofGetElapsedTimeMillis() - startTime;
if(timer >= endTime && !bTimerReached) {
bTimerReached = true;
dotFbo.begin();
ofClear(255,255,255);
dotFbo.end();

just store the current time in a variable whenever the cycle starts and compare that to ofGetElapsedTimeMillis every frame instead of getting the absolute value. you can reset that timer but it’s not very good idea since you might want to use it for something else with a different frequency.