Is it not an option to insert the sorting directly to the WP_Query class? It has all the parameters for handling this kind of situation http://codex.wordpress.org/Class_Reference/WP_Query#Custom_Field_Parameters
If you want to do that in the code, you can first validate all the dates, then convert them to the standard ISO(yyyy-mm-dd) format & then use the standard php functions to sort them like sort, ksort, (usort combined with strcmp) or any other depending on your data