ts = this.editingDigits.slice( 1 ) + digit;
        this.updateEditingDisplay();
      }

      removeDigit() {
        this.editingDigits = '0' + this.editingDigits.slice( 0, 3 );
        this.updateEditingDisplay();
      }

      updateEditingDisplay() {
        const minutes = this.editingDigits.slice( 0, 2 );
        const seconds = this.editingDigits.slice( 2, 4 );
        this.display.value = `${minutes}:${seconds}`;
      }

      finishEditing() {
        this.display.setAttribute( 'readonly', 'tru