void setup()
{
  Serial.begin(9600);
  Serial.println("Will start sending millis values in 5 seconds...");
  delay(5000);
}

void loop()
{
  Serial.println(String(millis()));

  if (Serial.available() > 0)
  {
    if (Serial.read() == '@')
    {
      Serial.println("Rebooting. . .");
      delay(100); // Give the computer time to receive the"Rebooting. . ." message, or it won't show up
      void (*reboot)(void) = 0; // Creating a function pointer to address 0 then calling it reboots the board.
      reboot();
    }
  }
}

Will start sending clock values in 5 seconds...
5000
5000
5000
5001
5001
5001
5002
5002
5002
. . .
6804
Rebooting...
Will start sending millis value in 5 seconds...
5000
5000
5000
5001
5001
5001
5002
5002
5002
5003
5003
. . .

#include <stdio.h>

#define STAMP_INSIDE 0
#define STAMP_BELOW -1
#define STAMP_ABOVE 1
#define STAMP_BEYONDO -1

    /* Intentionally very small, for fast wraparound
    ** Intentionally signed to stress test the logig.
    */
typedef signed char St

    /* fake clock, returns incrementing value, but folds around
    */
Stamp fakemillis(void)
{
static Stamp ticker =0;
return ticker++;
}

/* Check if"test" is inside or above/below the interval {low,high}
** low and high may have been wrapped around zero (--> low > high)
** return
**      0 :="test" inside interval
**      1 :="test" below interval
**      -1 :="test" above interval (but wrapped)
** The two impossible cases return -2.
*/
static int check_interval(Stamp low, Stamp high, Stamp test)
{
switch (4 *(high >= low)
        +2 *(test >= low)
        +1 *(test > high)
        ) {
        case 0: return STAMP_INSIDE;    /* inside (wrapped) */
        case 1:                 /* outside (wrapped) */
                return ((Stamp)(low - test) < (Stamp)(test - high)) ? STAMP_BELOW : STAMP_ABOVE;
        case 2: break;      /* impossible */
        case 3: return STAMP_INSIDE;    /* inside (wrapped) */
        case 4:                 /* all below */
                return ((Stamp)(low - test) < (Stamp)(test - high)) ? STAMP_BELOW : STAMP_ABOVE;
        case 5: break;      /* impossible */
        case 6: return STAMP_INSIDE;    /* inside normal case */
        case 7:                 /* all above) */
                return ((Stamp)(low - test) < (Stamp)(test - high)) ? STAMP_BELOW : STAMP_ABOVE;
        }  
return STAMP_BEYONDO;
}  

    /* Get new clock value, test if it is inside interval {*old, *old+width)
    ** iff inside: return STAMP_INSIDE;
    ** iff above (or below) return STAMP_ABOVE or STAMP_BELOW
    ** and UPDATE *old
   */
static int test_or_set(Stamp *old, Stamp width)
{
Stamp tick;
int diff;
tick = fakemillis();

diff = check_interval( *old, *old+width, tick);
if (!diff) return 0;
*old = tick;
return diff;
}


int main(void) {
Stamp goodlast=0;
Stamp tick=0;
Stamp badlast=0;
int goodtest;
int badtest;
unsigned uu;

for (uu = 0; uu < 260; uu++) {
    tick= fakemillis();
    if (tick > badlast+10) { badlast=tick; badtest=1; } else {badtest =0;}
    goodtest = test_or_set ( &goodlast, 10);
    printf("%x:Tick=%x bad=%x, badtest=%d good=%x goodtest=%d\
"
    , uu, (unsigned) tick
    , (unsigned) badlast, badtest
    , (unsigned) goodlast, goodtest
    );
    }
return 0;
}

#define CMD_SET_SPEED"S"
...
/* Internal configuration */
if(buffer.substring(0,1)==CMD_SET_SPEED) {
  updateSpeed(buffer.substring(1));
  }

int test_and_set_if_beyond( unsigned long *pprev, unsigned long width )
{
unsigned long tick, low,high;
low = *pprev;
high = low+width;
tick = millis();

if (low < high) {
    if (tick >= low && tick < high ) return 0; /* normal case */
    }
else { /* interval is wrapped , clock could have wrapped */
    if (tick >= low || tick < high) return 0;
    }
*pprev = tick;
return 1;
}

if (test_and_set_if_beyond ( &lightTimer, lightnessCheckPeriod)) {
    int newLightness = analogRead(brightnessPin);
    if(newLightness-lightness > LIGHT_TRESHOLD) {
      say(RESPONSE_FLASH);
    }
    lightness = newLightness;
  }
  if (test_and_set_if_beyond ( &pingTimer, pingTimerPeriod)) {
    say(RESPONSE_PING);
  }
  if (test_and_set_if_beyond ( &pingLEDTimer, pingTimerPeriod*2)) {
    digitalWrite(failPin, HIGH);
  }
  feed();