Made a real time timer but it updates too sporadically/weirdly

So, made a simple mobile game that has a real time timer. It will show how much time elapsed (to the second) from the moment you started a session. You can be running a session even if the app is closed or the phone is asleep.

The first version of the code would save the session start time. Then it would use a coroutine to add up the seconds in integers as they passed. (The clock starts at 0 at the beginning of the session.)

The problem with this is that the coroutine would stop when the player's device is put to sleep. So instead of calculating it that way, I decided the clock would display the time elapsed between the current time and start time for each second, again using a coroutine.

Now the issue is that the timer displays weirdly. The seconds seem(?) to go faster than they're supposed to.

Here is the code:

 public class Timer : MonoBehaviour
 {
     public TMP_Text timerText;
     public DateTimeValue timeStarted;
 
     public IEnumerator RunTimer()
     {
         while (true)
         {
             var rightNow = DateTime.Now;
             TimeSpan timeElapsed = rightNow - timeStarted.value;
             DisplayTimer(timeElapsed);
             yield return new WaitForSeconds(1f);
         }
     }
 
     public void DisplayTimer(TimeSpan timeElapsed)
     {
   
         timerText.text =
             $"{timeElapsed.Days} days\n" +
             $"{timeElapsed.Hours} hours\n" +
             $"{timeElapsed.Minutes} minutes\n" +
             $"{timeElapsed.Seconds} seconds\n";
 
     }
 }

Appreciate any help on how to get the display to consistently update the current number of seconds elapsed, instead of have a jerky and inconsistent output.