Can your independent programmer supply the code for your program? If not, we cannot see what changes may have been made to make the code work in your system, a simple date field issue such as this sounds like a programmer either wanted to be sure you would call back in 2016, or someone severely "humped the bunk" as it were. In either case, the only way we can even start to resolve this is with a look at the raw source code. Do you have that available?
It is possible, but not without a look at your code. If you cannot get it, then unfortunately there is nothing we can do, short of dialing into your computer, and that would likely prove ineffective, and cost you more than you have already invested in this question!
I personally do not have a great deal of time for that project, but I am sure anyone who wants it can make you an offer on that one! I will go ahead an opt out to give them a chance! :)
I have charged up to $500 USD myself, good luck! :)