// PropertyChanged Event Handler
    private void thirdLevel_PropertyChanged(object? sender, PropertyChangedEventArgs e)
    {
        // other relevant updates on thirdLevel object's PropertyChanged Event
        ...

        // *** Update StartTime value of subsequent Items in the Collection ***
        if (e.PropertyName == "Duration")
        {
            var index = ThirdLevels.IndexOf(sender as ThirdLevelViewModel);
            for (int i = index + 1; i < ThirdLevels.Count...
        }
    }

It's super easy, any problem?