I intend to make the device runs about couple hours a day(let say 2hrs) -> go to deep sleep -> wake up in next 7 days -> repeat the process.

I got the code from one of member here (DotPi) which I can able to make the device wake up every 10 seconds and then back to sleep. When it wakes up, the LED is flashing.

My question is how to make the LED flashes for certain amount of time. For example from 7pm to 8 pm. After that the device should go to sleep again and wait for the next alarm triggered.

#include <RTClibExtended.h>
#include <Wire.h>
#include <LowPower.h>
#include <stdlib.h>
#include <avr/interrupt.h>      // library for interrupts handling
#include <avr/sleep.h>          // library for sleep
#include <avr/power.h>          // library for power control


#define wakePin 2    //use interrupt 0 (pin 2) and run function wakeUp when pin 2 gets LOW
#define ledPin 13

RTC_DS3231 rtc;
char daysOfTheWeek[7][12] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
int interval_sec=10; //An alarm every 10 sec
int i;

void setup () {

  Serial.begin(9600);

  delay(3000); // wait for console opening

  /**RTC**/
  if (! rtc.begin()) {
    Serial.println("Couldn't find RTC");
    while (1);
  }
  rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));

  /**INTERRUPT**/
  //Set pin D2 as INPUT for accepting the interrupt signal from DS3231
  pinMode(wakePin, INPUT);
  Wire.begin();
  //clear any pending alarms
  rtc.armAlarm(1, false);
  rtc.clearAlarm(1);
  rtc.alarmInterrupt(1, false);
  rtc.armAlarm(2, false);
  rtc.clearAlarm(2);
  rtc.alarmInterrupt(2, false);

  //Set SQW pin to OFF (in my case it was set by default to 1Hz)
  //The output of the DS3231 INT pin is connected to this pin
  //It must be connected to arduino D2 pin for wake-up
  rtc.writeSqwPinMode(DS3231_OFF);

  Serial.println("Initialisation complete.");
  delay(100); //Allow for serial print to complete.
}

void loop () {
    DateTime now = rtc.now();

    Serial.print(now.year(), DEC);
    Serial.print('/');
    Serial.print(now.month(), DEC);
    Serial.print('/');
    Serial.print(now.day(), DEC);
    Serial.print(" (");
    Serial.print(daysOfTheWeek[now.dayOfTheWeek()]);
    Serial.print(") ");
    Serial.print(now.hour(), DEC);
    Serial.print(':');
    Serial.print(now.minute(), DEC);
    Serial.print(':');
    Serial.print(now.second(), DEC);
    Serial.println();

    /**INTURRUPT**/
    DateTime nextAlarm = now + TimeSpan(0, 0, 0, interval_sec);
    Serial.print("Alarm at:");
    Serial.println(nextAlarm.hour());
    Serial.println(nextAlarm.minute());
    Serial.println(nextAlarm.second());
    rtc.setAlarm(ALM1_MATCH_HOURS, nextAlarm.second(), nextAlarm.minute(), nextAlarm.hour(), 1);   //set your wake-up time here
    rtc.alarmInterrupt(1, true);
    attachInterrupt(0, wakeUp, LOW);       //use interrupt 0 (pin 2) and run function wakeUp when pin 2 gets LOW    
    LowPower.powerDown(SLEEP_FOREVER, ADC_OFF, BOD_OFF);   //arduino enters sleep mode here
    detachInterrupt(0);                                    //execution resumes from here after wake-up
    //When exiting the sleep mode we clear the alarm
    rtc.armAlarm(1, false);
    rtc.clearAlarm(1);
    rtc.alarmInterrupt(1, false);
  
    while ( now.minute()>0 && now.minute() < 30)
    {
       digitalWrite(ledPin, HIGH);
       delay(100);
       digitalWrite(ledPin, LOW);
       delay(100);
    }

}

void wakeUp()        // here the interrupt is handled after wakeup
{
}

I was trying to use the while loop for that but the LED always flash and never go back to sleep :(