Read date from a file and rename two files using the date
Important change

I would suggest you look at the Path class. A lot of your logic is about manipulating filename and this class does it all for you.

Less important change

Depending on how big the file is, doing File.ReadAllLines might take more memory than needed. Reading one line at a time would be better.

GetFilesInDirectory function name isn't true. It also renames the file.

You could use a bit of error checking. For example, what happen if the string "exif:DateTimeOriginal=" can't be found in the file. What if there's no xmp file to the nef file.