Question about millis()

i'm using code right led display using millis.
my question happen after approximately 50 days.

the code :

/*

*/
#include <leddisplay.h>

// define pins led display.
// can change these, re-wire board:
#define datapin 6              // connects display's data in
#define registerselect 7       // display's register select pin
#define clockpin 8             // display's clock pin
#define enable 9               // display's chip enable pin
#define reset 10               // display's reset pin
#define displaylength 8        // number of bytes needed pad string
// create instance of led display:
leddisplay mydisplay = leddisplay(datapin, registerselect, clockpin,
enable, reset, displaylength);

int lcdstate = 0; //estado lcd
long previousmillis = 0;
long interval = 1000;
int brightness = 15;        // screen brightness
char open[] = {' ','o','p','e','n',' ',' ',' '};
char door[] = {' ','d','o','o','r',' ',' ',' ',};
//char del[]={' ',' ',' ',' ',' ',' ',' ',' ',};
void setup() {
 serial.begin(9600);

 // initialize display library:
 mydisplay.begin();
 mydisplay.setstring("open");
 mydisplay.home();
 mydisplay.setbrightness(brightness);
}

void loop() {
     if (millis()- previousmillis > interval) {
       previousmillis = millis();
       if (lcdstate == 0){
           mydisplay.home();
           for (int thisposition = 0; thisposition < 8; thisposition++)
             {
              mydisplay.write(open[thisposition]);
             }
           lcdstate = 1;
       }
       else {    
           mydisplay.home();
           for (int thisposition = 0; thisposition < 8; thisposition++)
            {
             mydisplay.write(door[thisposition]);
            }
           lcdstate = 0;
       }
       
}
}

change this...

code: [select]

long previousmillis = 0;
long interval = 1000;

...to this...

code: [select]

unsigned long previousmillis = 0;
unsigned long interval = 1000;

...and should go.  i believe can test using "unsigned short" instead of "unsigned long".  the roll-over should occur in on 1 minute.

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