Arduino code sample

/* This code sample is associated with the following question on Stack Exchange:
 * http://arduino.stackexchange.com/questions/20919/arduino-nano-pwm-output-acting-strange-after-running-for-a-day-or-so
 */

#include <Time.h>

#define _analogWrite(a, b) analogWrite(a, 255-(b))
#define clamp(X, minVal, maxVal) min(max(X, minVal), maxVal)

const PROGMEM int pwmPin = 3;
time_t lastUpdate = 0;

inline void flushSerialBuffer() {
    while (Serial.available()) Serial.read();
}

void setup() {
    Serial.begin(9600);
    Serial.setTimeout(500);
    pinMode(pwmPin, OUTPUT);
    _analogWrite(pwmPin, 0);
}

void loop() {
    time_t currentTime = now();

    if (Serial.available()) {
        delay(500); // Just to be super safe
        int pwmValue = Serial.parseInt();
        flushSerialBuffer();
        if (pwmValue < 0 || pwmValue > 255) pwmValue = 0;
        _analogWrite(pwmPin, pwmValue);
        lastUpdate = currentTime;
    }

    if (currentTime - lastUpdate > 30) _analogWrite(pwmPin, 0);

    delay(250);
}