Stopwatch fails to reset - arduino-uno

Question

I am working on an Arduino stopwatch, where it needs a start, stop, and reset button. To reset it, I am using a variable called starttime that is updated to equal to millis(), and then take the difference to display time. However, past thirty seconds, the starttime does not update correctly, and the resulting difference between starttime and millis() is equivalent to 65 seconds. Can someone explain why this is happening?
#include <LiquidCrystal.h>
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);
const int start = 8;
const int stp = 9;
const int reset = 10;
int seconds;
int minutes;
int timedif;
int starttime = -1;
int timestall = 0;
bool onoff = true;
void setup()
{
pinMode(start, INPUT);
pinMode(stp, INPUT);
pinMode(reset, INPUT);
lcd.begin(16, 2); //Initialize the 16x2 LCD
lcd.clear(); //Clear any old data displayed on the LCD
Serial.begin(9600);
}
void loop()
{
int bsta = digitalRead(start);//sees if start switch is pressed
int bstp = digitalRead(stp);//records if stop switch is pressed
int bres = digitalRead(reset);//records if reset switch is pressed
lcd.setCursor(0,0);
lcd.print("Stopwatch");//prints stopwatch top row
lcd.setCursor(0,1);
if (starttime == -1) { //if running first time, time dif is initiated
starttime = millis();
}
timedif = (millis() - starttime )/1000 + timestall; //will get difference in terms of seconds
minutes = floor(timedif/60); // divides by sixty, drops decimal
seconds = timedif%60; //gets remainder of divided by 60
lcd.print(minutes);
lcd.print(":");
lcd.print(seconds);
if (seconds < 10) {
lcd.setCursor(3,1);
lcd.print(' ');
}
if (bstp == HIGH) {
onoff = false;
while(onoff == false) {
if (digitalRead(start) == HIGH) {
onoff = true;
}
}
timestall = timedif;
starttime = millis();
}
if (bres == HIGH) {
delay(100);
timestall = 0;
starttime = millis();
timedif = 0;
lcd.clear();
Serial.println("stall:");
Serial.println(timestall);
Serial.println("dif");
Serial.println(timedif);
Serial.println("start");
Serial.println(millis() - starttime);
}
}

Answer 1

You should use long or unsigned long instead if int to declare variables that hold time values.
A int variable can only hold 32,767 millisecond hence the 32 sec. where a long variable can hold 2,147,483,647 millisecond which is something like 48 days. A unsigned long can hold double of that but can not hold a negative value.