Use millis or micros if you need more precise timing.

Here is an example sketch:

long lastDoTime = 0;

void setup(){
    Serial.begin(115200);
    delay(1000);
    Serial.println("Hello! We will do something at every ms");
}

void doThisAtEvery(int ms){
    if( millis() - lastDoTime >= ms ){
        // Must save the lastDoTime
        lastDoTime  = millis();

        // Do some stuff at every ms
    }
}

void loop(){
    // It will do the thing in every 100 ms.
    doThisAtEvery(100);
}

If you want to toggle a pin let's say every 100 microsec

long lastToggleTime = 0;
int motorPin = 14;
boolean lastPinState = LOW;

void setup(){
    Serial.begin(115200);
}

void togglePinEvery(int micros){
    if( micros() - lastToggleTime >= micros ){
        lastToggleTime = micros();
        digitalWrite(motorPin,!lastPinState);
        lastPinState = !lastPinState;
    }
}

void loop(){
    togglePinEvery(100);
}

EDIT Since you wanted timers only.

Here is a detailed explanation about timers: https://techtutorialsx.com/2017/10/07/esp32-arduino-timer-interrupts/

Code example:

volatile int interruptCounter;
int totalInterruptCounter;
 
hw_timer_t * timer = NULL;
portMUX_TYPE timerMux = portMUX_INITIALIZER_UNLOCKED;
 
void IRAM_ATTR onTimer() {
  portENTER_CRITICAL_ISR(&timerMux);
  interruptCounter++;
  portEXIT_CRITICAL_ISR(&timerMux);
 
}
 
void setup() {
 
  Serial.begin(115200);
 
  timer = timerBegin(0, 80, true);
  timerAttachInterrupt(timer, &onTimer, true);
  timerAlarmWrite(timer, 1000000, true);
  timerAlarmEnable(timer);
 
}
 
void loop() {
 
  if (interruptCounter > 0) {
 
    portENTER_CRITICAL(&timerMux);
    interruptCounter--;
    portEXIT_CRITICAL(&timerMux);
 
    totalInterruptCounter++;
 
    Serial.print("An interrupt as occurred. Total number: ");
    Serial.println(totalInterruptCounter);
 
  }
}

Hi Mate I suggest to use the Network Time Protocol. It’s a standard Internet Protocol (IP) for synchronizing the computer clocks to some reference over a network. The protocol can be used to synchronize all networked devices to Coordinated Universal Time (UTC). NTP sets the clocks of computers to UTC, any local time zone offset or day light saving time offset is applied by the client. In this manner clients can synchronize to servers regardless of location and time zone differences. The server is using the User Datagram Protocol (UDP) on port 123. A client then transmits a request packet to a NTP server. In response to this request the NTP server sends a time stamp packet.

Please try this code:

#include "NTPClient.h"
#include "ESP8266WiFi.h"
#include "WiFiUdp.h"

const char *ssid = "***********";
const char *password = "***********";

const long utcOffsetInSeconds = 0;

char daysOfTheWeek[7][12] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};

// Define NTP Client to get time
WiFiUDP ntpUDP;
NTPClient timeClient(ntpUDP, "pool.ntp.org", utcOffsetInSeconds);

void setup(){
Serial.begin(115200);

WiFi.begin(ssid, password);

while ( WiFi.status() != WL_CONNECTED ) {
delay ( 500 );
Serial.print ( "." );
}

timeClient.begin();
}

void loop() {
timeClient.update();

Serial.print(daysOfTheWeek[timeClient.getDay()]);
Serial.print(", ");
Serial.print(timeClient.getHours());
Serial.print(":");
Serial.print(timeClient.getMinutes());
Serial.print(":");
Serial.println(timeClient.getSeconds());
//Serial.println(timeClient.getFormattedTime());

delay(1000);
}

Note: the line:

const long utcOffsetInSeconds = 0;

sets the UTC time (basically the Greenwich time with no light saving time offset), if you wish, set your own offset in seconds from the Greenwich time. Great article here: https://www.instructables.com/Getting-Time-From-Internet-Using-ESP8266-NTP-Clock/

If you get two readings and you make the difference between the two, you have the duration of the operation and, (very important since we are talking about network operations), the time reference for a following analysis.

I hope this helps All the best

Its actually surprisingly hard to estimate battery life, especially over long periods. Battery voltage is not linear, meaning that it will suddenly drop off as it gets close to "empty". This also depends on the chemistry of the battery. Like others have said, you can use voltage to try and guess where the battery is at, but its not an exact science. You might need to characterize your battery and create a lookup table to check the level.

You can also try to estimate overall battery life by measuring your average current draw and checking that against the mAh rating of your battery.