Replace a substring by a rxjs countdowntimer

Question

I am trying to implement a countdowntimer using rxjs in my angular 12 application. In my ts I have :

let timeLeft$ = interval(1000).pipe(
  map(x => this.calcTimeDiff(orderCutOffTime)),
  shareReplay(1)
);

calcTimeDiff() will return seconds, minutes and hours;

I have to create a string out of this and display that in the HTML. Basically replace an existing word in the string with timeLeft$ which will be a countdowntimer to be shown in the HTML.

Something like : this.orderCutOffMessage = someString.replace('Down', timeLeft$)

https://henrikmassow.medium.com/implement-a-countdown-timer-with-rxjs-in-angular-61600d1af00c

orderCutOffTime = "1645567200000" // Unix timestamp in milliseconds

private calcTimeDiff(cutOffTime: any): timeComponents {
const finalCutOffTime = (cutOffTime).valueOf();
const milliSecondsInASecond = 1000;
const hoursInADay = 24;
const minutesInAnHour = 60;
const secondsInAMinute = 60;
const timeDifference = finalCutOffTime - Date.now();

const hoursToFinalCutOffTime = Math.floor(
  (timeDifference /
    (milliSecondsInASecond * minutesInAnHour * secondsInAMinute)) %
  hoursInADay
);

const minutesToFinalCutOffTime = Math.floor(
  (timeDifference / (milliSecondsInASecond * minutesInAnHour)) %
  secondsInAMinute
);

const secondsToFinalCutOffTime =
  Math.floor(timeDifference / milliSecondsInASecond) % secondsInAMinute;


return { secondsToFinalCutOffTime, minutesToFinalCutOffTime, hoursToFinalCutOffTime };

}