如標題所述我使用millis()來計時，我是打算用來計算設備開啟的時間，我的作法是如果設備開啟就抓取當前的millis()，設備關閉時在抓取一次millis()，之後再用關閉的millis()減掉開啟時的millis()來算出開啟多久，可是我測試不管開多少秒開啟前的millis()跟開啟後的millis()都只相差一點多秒，請問這個可能是甚麼原因？

硬體是使用esp32
以下是我的程式碼，這裡只有把抓到前後的millis()送到資料庫，計算的部分在其他程式，不過重點是上傳到資料表的數值都只差一秒多，有問題的部分是在Motor==on或off判斷的地方

#include <WiFi.h>
#include <ArduinoJson.h>
//#include <analogWrite.h> 



#define light 35 //光敏電阻
//#define water_sensor 36 // 水位感測器
#define soil_sensor 33 //土壤濕度感測器
#define fannow 14 //通風風扇
#define watervalmotor 26 //抽水馬達(容器加水)
#define motornow 27 //抽水馬達(澆水)

/**************超音波測距模組*****************/
#define trigPin 5 //超音波測距模組
#define echoPin 18 //超音波測距模組
//define sound speed in cm/uS
#define SOUND_SPEED 0.034
#define CM_TO_INCH 0.393701

long duration;
//float distanceCm;
float watervalue;
float distanceInch;

/*******************************************/

String Motor; //抽水馬達(澆水)變數
String WaterValMotor; //抽水馬達(容器加水)變數
String fan_cool; //通風風扇 變數
int Soilhumidity;//土壤濕度
int soilValue;
int soilMoisture;//定義土壤濕度
int watervalueint;
unsigned long MotorOpenTime=0; //澆水時間計算
unsigned long MotorOFFTime=0;
unsigned long MotorTime=0;
float opentimeF=0;
float timewatervalue=0; //秒數X水量
int MotorOff;



HardwareSerial myHardwareSerial(1);
static unsigned int co2 = 0;
static unsigned int ucRxBuffer[10];
 
const unsigned long HTTP_TIMEOUT = 10000;
const char* ssid = "myssid";
const char* password = "mypassword";

const char* host = "192.168.0.17";
const char* webpage= "sensor_control/Status_field2.php";
const char* webpageupdate= "sensor_control/AddData2.php?";
WiFiClient client;

void setup(){
  pinMode(fannow, OUTPUT);
  digitalWrite(fannow, HIGH);
  pinMode(watervalmotor, OUTPUT);
  digitalWrite(watervalmotor, HIGH);
  pinMode(motornow, OUTPUT);
  digitalWrite(motornow, HIGH);
  pinMode(light, INPUT);
  pinMode(soil_sensor,INPUT);
  //pinMode(water_sensor,INPUT); //設置water_sensor對應的腳GPIO36為輸入
  pinMode(trigPin, OUTPUT); // Sets the trigPin as an Output
  pinMode(echoPin, INPUT); // Sets the echoPin as an Input
  Serial.begin(115200);
  delay(500);
  Serial1.begin(115200);
  myHardwareSerial.begin(9600, SERIAL_8N1, 12, 13); //DS-Co2-20 GPIO Serial的TX,RX
  Serial.println();
  
  Serial.printf("Connecting to %s ", ssid);
  WiFi.begin(ssid, password);
  delay(500);
  while (WiFi.status() != WL_CONNECTED)
  {
    delay(500);
    Serial.print(".");
  }
  Serial.println(" connected");
}

void loop(){
  
  /***********************DS-CO2-20 Sensor**********************/
  static uint32_t lasttime=millis();

  byte request[] = {0x42, 0x4d, 0xe3, 0x00, 0x00, 0x01, 0x72};
  Serial1.write(request, 7);
  delay(500);
  while (Serial1.available()){
    for(int i=0; i<12; i=i+1){
      ucRxBuffer[i]=Serial1.read();
      }
    co2 = ucRxBuffer[4]*256+ucRxBuffer[5];
    Serial.print("ppm:");
    Serial.println(co2);
    
    }
/********************** 光敏電阻 ********************************/
  int Light=analogRead(light); // 0 ~ 4096
  //Serial.println(Light);
  int LightVal = map(Light,0,4096,0,100); //換算 0~4096 成0~100
  Serial.print("光照：");
  Serial.println(LightVal);
  delay(100);
/************************** 土壤感測器 ********************************/

  soilValue = analogRead(soil_sensor); //讀取土壤感測器上的電壓值
  Serial.print("土壤濕度:");
  Serial.println(soilValue);
  soilMoisture = map(soilValue,0,4096,100,0);//把電壓值按照[0,1023]映射到[100,0]

/******************************* 超音波測距模組(水位感測) ************************************/
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  // Sets the trigPin on HIGH state for 10 micro seconds
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  // Reads the echoPin, returns the sound wave travel time in microseconds
  duration = pulseIn(echoPin, HIGH);
  
  // Calculate the distance
  //distanceCm = duration * SOUND_SPEED/2;
  watervalue = duration * SOUND_SPEED/2;
  
  // Convert to inches
  //distanceInch = distanceCm * CM_TO_INCH;
  
  // Prints the distance in the Serial Monitor
  Serial.print("watervalue (cm): ");
  Serial.println(watervalue);
  //Serial.print("Distance (inch): ");
  //Serial.println(distanceInch);
/*********************************************************************************/

  if (client.connect(host, 80)){
    Serial.println("[Host connected]");
    //Get the LED color
    Serial.println("[Sending a request as:]");
    client.print(String("GET /")  + webpage + " HTTP/1.1\r\n" +
                 "Host: " + host + "\r\n" +
                 "Connection: close\r\n" +
                 "\r\n"
                );
    Serial.println(String("GET /")  + webpage + " HTTP/1.1\r\n" +
                 "Host: " + host + "\r\n" +
                 "Connection: close\r\n" +
                 "\r\n");
                  
    int timeoutcnt=0;
    while((!client.available()) && (timeoutcnt < 50)){
       delay(200);  //Use this with time out
       timeoutcnt++;
    }
        //https://circuits4you.com/2017/12/09/thing-speak-esp8266/  
        //---------------------------------------------------------------------
        //If data is available before time out read it.
    if(timeoutcnt < 50){ // Check HTTP status
      char status[32] = {0};
      Serial.print(client);
      client.readBytesUntil('\r', status, sizeof(status));
      for (int i=0;i<32;i++)
        Serial.print((char)status[i]);
      Serial.println();
      // It should be "HTTP/1.0 200 OK" or "HTTP/1.1 200 OK"
      if (strcmp(status + 9, "200 OK") != 0){
        Serial.print(F("Unexpected response: "));
        Serial.println(status);
        return;
      }

      // Skip HTTP headers
      char endOfHeaders[] = "\r\n\r\n";
      if (!client.find(endOfHeaders)) {
        Serial.println(F("Invalid response"));
        return;
      }

      // Allocate the JSON document
      // Use arduinojson.org/v6/assistant to compute the capacity.
      const size_t capacity = JSON_ARRAY_SIZE(1) + JSON_OBJECT_SIZE(2) + JSON_OBJECT_SIZE(4) + JSON_OBJECT_SIZE(10) + 314;
      DynamicJsonDocument jsondoc(capacity);

     // Parse JSON object
      DeserializationError error = deserializeJson(jsondoc, client);
      if (error) {
        Serial.print(F("deserializeJson() failed: "));
        Serial.println(error.c_str());
        return;
      }
      client.stop();
      Serial.println("[Disconnected]");
      // Extract value
      String temperature = jsondoc[0]["Temp"];
      Serial.print("Temperature = ");  
      Serial.println(temperature.toFloat());
      String humidity = jsondoc[0]["Humidity"];
      Serial.print("Humidity = ");  
      Serial.println(humidity.toFloat());
      String Soilhumidity = jsondoc[0]["Soilhumidity"];
      Serial.print("Soilhumidity = ");
      Serial.print(Soilhumidity.toInt());
      Serial.println("%");
      String Motor = jsondoc[0]["Motor"];
      Serial.print("Motor = ");
      Serial.println(Motor);
      String Co2 = jsondoc[0]["Co2"];
      Serial.print("Co2 = ");
      Serial.println(Co2);
      String fan_cool = jsondoc[0]["fan_cool"];
      Serial.print("fan_cool = ");
      Serial.println(fan_cool);
      String WaterValMotor = jsondoc[0]["WaterValMotor"];
      Serial.print("WaterValMotor = ");
      Serial.println(WaterValMotor);
      String WaterValueHigh = jsondoc[0]["WaterValueHigh"];
      Serial.print("WaterValueHigh = ");
      Serial.println(WaterValueHigh);

/***********************************/
      watervalueint = WaterValueHigh.toInt();
      watervalue = watervalueint - watervalue;
      Serial.print("水位高度 = ");
      Serial.println(watervalue);

/****************************************設備開關判斷****************************************/
      //timewatervalue = 0.0;
      MotorOpenTime = 0;
      
      if(Motor=="on"){
        digitalWrite(motornow, LOW);
        MotorOpenTime = millis();
        Motor = "standby";
        
      }else if(Motor=="off"){
        digitalWrite(motornow, HIGH);
        MotorOFFTime = 0;
        MotorOFFTime = millis();
        Motor = "end";
         
      }

      if(fan_cool == "on"){
        digitalWrite(fannow, LOW);
        //fan_cool = "on";
      }else if(fan_cool == "off"){
        digitalWrite(fannow, HIGH);
        //fan_cool = "off";
      }

 
      delay(900);
      /********************************************/
      if (client.connect(host, 80)){
        String dbserver = host;
        String getStr = "GET /sensor_control/AddData2.php?";
        getStr +="Co2=";
        getStr += String(co2);
        getStr +="&LightVal=";
        getStr += String(LightVal);
        getStr +="&Soilhumidity=";
        getStr += String(soilMoisture);
        if(Motor == "off" or Motor == "end"){
          getStr +="&Motor=";
          getStr += String("standby");
        }
        getStr +="&MotorOpenTime=";
        getStr += String(MotorOpenTime);
        getStr +="&MotorOFFTime=";
        getStr += String(MotorOFFTime);
        getStr +="&WaterValue=";
        getStr += String(watervalue);
        /*getStr +="&TimeWaterValue=";
        getStr += String(timewatervalue);*/
        getStr += " HTTP/1.1\r\n";
        getStr += "Host: " + String(dbserver) \
        + "\r\n"+ "Connection: close\r\n\r\n";
        Serial.println(getStr);
        client.print(getStr);
        delay(10);
        client.stop();
      }
        /*******************************************/
      
      
    


/***********************************************************************************/
      }else{
       Serial.println("Request timeout..");
       client.stop();           
       return;
    }
  }
  else{
    Serial.println("connection failed!]");
    client.stop();
  }
    
  //delay(5000);
}