//lastFrameTime is an integer, which works as a boolean in C.
if(!lastFrameTime){
    lastFrameTime = glutGet(GLUT_ELAPSED_TIME);
}
int now = glutGet(GLUT_ELAPSED_TIME);
int elapsedMilliseconds = now - lastFrameTime;
float deltaTime = elapsedMilliseconds / 1000.0f;
lastFrameTime = now;