Photoresistor - Re-calibration

const int PhotoR = A0;
const int LEDpin = 6;
int ReadValue = 0;
unsigned long Time;
int MinVal = 1000;
int MaxVal = 0;
const int Button = 3;
int ButtonVal = 0;
boolean check = false;

void setup()
{
  pinMode (PhotoR, INPUT);
  pinMode (LEDpin, OUTPUT);
  pinMode (Button, INPUT);
  Serial.begin(9600);
}

void calibrate()
{
  Serial.println("in calibrate");
  digitalWrite (LEDpin, HIGH);
  Time = millis();
   while (millis() - Time <=5000)
  {
    ReadValue = analogRead(PhotoR);
    if (ReadValue < MinVal)
    {
      MinVal = ReadValue;
    }
    if (ReadValue > MaxVal)
    {
      MaxVal = ReadValue;
    }
  }
  digitalWrite(LEDpin, LOW);
}


void loop()
{
  ButtonVal = digitalRead (Button);
  if (check)
  {
    Serial.println("Check is true");
    if (ButtonVal == 1)
    {
      Serial.println("Button is on");
      calibrate();
    }
  }
  else
  {
    Serial.println("Check is false");
    calibrate();
    check = true;
  }
  ReadValue = analogRead(PhotoR);
  ReadValue = map(ReadValue, MinVal, MaxVal, 255, 0);
  ReadValue = constrain(ReadValue, 0, 255);
  Serial.println(ReadValue);
  analogWrite(LEDpin, ReadValue);
}