Exéxcution stops or slows down when locking device or changing application

Hello,
I have a ionic mobile application that displays a chronometer.
When i launch the chronometer, it stops when i lock the device or when i change the application then return to the chronometer.
How can I make the chronometer continue normally please ?
Here is the chronometer code in my angular 4 component
Thank you in advance.

/**
 * chronometre
 * */
 chronometer(): void {
    if (this.startchron == 1) {
    this.chronoTimer.tenthOfSecond += 1;

  // set seconds
  if (this.chronoTimer.tenthOfSecond > 9) {
    this.chronoTimer.tenthOfSecond = 0;
    this.chronoTimer.seconds += 1;
  }

  // set minutes
  if (this.chronoTimer.seconds > 59) {
    this.chronoTimer.seconds = 0;
    this.chronoTimer.minutes += 1;
  }
  setTimeout(() => this.chronometer(), 100);
}

}