unsigned long zeroTime, splitTime, newTime, oldTime = 0, fivek = 4999; // needed 1ms for Serial printing
bool startFlag = 0;
int buttonPin = 2;
int motorPin = 3;

void setup() {
  Serial.begin(9600);
  pinMode(buttonPin, INPUT_PULLUP);
  pinMode(motorPin, OUTPUT);
}

void loop() {
  if (!digitalRead(buttonPin)) { // the button is press (LOW)
    zeroTime = millis(); // set zeroTime to the time of button press
    delay(150);
    oldTime = zeroTime; // use oldTime as a variable
    startFlag = 1; // this indicates the button was pressed
    Serial.print("Button pressed. \nSplit time of 0 seconds at ");
    Serial.println(zeroTime);
  }

  if (startFlag) { // if the button was pressed
    newTime = millis(); // start a new 5 seconds count up
    if (newTime - oldTime > fivek) { // if the 5 seconds interval has passed
      splitTime = (newTime - zeroTime)/1000; // time to do things
      Serial.print("Split time of ");
      Serial.print(splitTime); // show the split time, or turn on a motor
      Serial.print(" seconds at ");
      Serial.println(newTime);
      oldTime = newTime; // start a new split time
      digitalWrite(motorPin, !digitalRead(motorPin)); // change the motor state
    }
  }
}