I've built some custom form elements that are pretty much exactly the same to their base class but with an added property on, all seems good however when I tick the option to display the summary message with a list of all the form elements and their values before the message doesn't appear to pull through my custom DateTime elements value, all other custom elements work correctly though. I can see in the EPiServerFormsSamples.js that the showSummarizedText function is not returning this specific value.
Any ideas would be much appreciated.
For the custom element you need to calculate its value your self for displaying in summary text. You need override the method getCustomElementValue to return your element's value. You can find the method in EPiServerFormsSample.js.