Basically EXIF data is for photographers to troubleshoot their pictures..........it has no forensic relevance as it does not record amendments, and is not secure.
You can preserve some of the data such as the date, but you have to make a conscious effort to do this, and could never prove it.
It is nothing like the metadata that might be attached to a Microsoft Office document, for example. EDIT:
The dates seem particularly irrelevant. I downloaded the sample and it reverted to my system date. When I make a copy (save as) it gives the new image the current system date. That implies that any form of editing that would require a save will change the image EXIF date.
This does sort of make sense as EXIF is not a forensics tool. It is to assist with photo editing and troubleshooting. Obviously, in that situation you are only interested in the last settings that apply to the image you are currently looking at, as they are the ones that have produced it.
Given that World + dog can alter the clocks in their cameras and computers I don't see much forensics value in that image date. Also remember that photographs can be scanned to produce a digital image, so the image date might bear absolutely no relationship to the actual photographic event.