"Whenever I change the sort order by code and save the page, the publish event is triggered again, creating an infinite loop".

Solution 1: Just hook into the publishing event instead of published to set the property of the current page but dont call the save method.

Solution 2: The publish event will always triger whenever you publish the page either programmaticaly or in GUI. The only thing you can do is to create a bool property on the page something like "IsAlreadySorted" and on first publish event, set this up and any other subsequent events use this as a flag. To do this, you have to hook into publishing and published event both.

You can't obviously save the page in an event. But what you could do, is to subscribe to the PublishingContent (or PublishingPage) event instead. This event occurs before the page is persisted to the database, then there is no need to call the save method again. Why are you updating all children? Isn't it enough to just update the page that was published? Maybe I missunderstood something.

Here's an example on how you could do it (untested of course):

[InitializableModule]
[ModuleDependency(typeof(EPiServer.Web.InitializationModule))]
public class InitializationModule : IInitializableModule
{
    public void Initialize(InitializationEngine context)
    {
        var events = ServiceLocator.Current.GetInstance<IContentEvents>();

        events.PublishingContent += this.PublishingConcert;
    }

    public void Uninitialize(InitializationEngine context)
    {
        var events = ServiceLocator.Current.GetInstance<IContentEvents>();

        events.PublishingContent -= this.PublishingConcert;
    }

    private void PublishingConcert(object sender, ContentEventArgs e)
    {
        var concertPage = e.Content as ConcertPage;

        if (concertPage == null)
        {
            return;
        }

        concertPage[MetaDataProperties.PagePeerOrder] = int.Parse(concertPage.EventStartTime.ToString("yyyyMMddHHmmss"));
    }
}

Just convert the date to an integer.

Oh forgot, if you want to change the sort order you could just do:

int.Max - int.Parse(concertPage.EventStartTime.ToString("yyyyMMddHHmmss"))

Johan Petersson: I wanted to update all children so the sort order would be correct for all children. Instead of setting the sort order to myDate, I was oddly enough planning on setting the sort order to the list index of each page after sorting them. So page A = 1, B = 2 etc. Of course, using the date in integer form is much better. That way I only have to update the one page that's getting published. Thanks a lot!