Measuring time between two photoresistors and printing time on lcd
The most accurate approach is via In out capture on your timer.

Alternatively you can use an interrupt to start and stop the timer.

The worst approach is to poll 

However, phtoresistors are slow and generally inconsistent. This can lead to errors in reading. S phototransistor or diode works better.

Edit: it is crazy to try to read the signal in analog. Instead design the circuitry so that it breaches the digital low or high thresholds so you can use digitalRead instead.

You can simply use millis or micros to time stamp the events and calculate the difference.

If you want more precise timing measurements, Google ticks implementation.

edit 2:

here is what I tried:

if (dtime_running==0) {             //event 1 has not arrived
    if (EVENT_ON(EVENT1_PIN)) {     //event 1 has arrived
        dtime = time_now();         //time stamp the event
        dtime_available = 0;        //data not yet available
        dtime_running = 1;          //time measurement running
    }
} else {                            //event 1 has arrived and now wait for event 2
    if (EVENT_ON(EVENT2_PIN)) {     //and event 2 has arrived
        dtime = time_now() - dtime; //calculate the timing difference
        dtime_available = 1;        //data now available
        dtime_running = 0;          //stop taking measurement
    } 
}


On two pulses that are 500ms apart, here is the measurement:


Notice that in the box on the upper right, dtime has a value of 500, just as we expected.

the risk with this approach is that if the loop has other tasks, jitters to the measurement will show up. but for slow events, or long time measurements, it may not matter that much.