Hey there, I'm really interested in working on this piece of software for you. I am a programmer with quite a few years of experience and also have a Masters Degree in Computer Science. I enjoy programming and also have a geospatial background. I've dealt with data similar to yours for both updating, reading, storing and visualizing. So I'm very interested in this project.
I've read the ICD and looked at the GPS binary files to understand the format better. This is an application that I could develop for you no problem. I'm a C++/C# developer and I can develop software on both Windows and Linux. Let me know which operating system you are interested in using and I'll develop the software for that OS.
I think that the best way to do this is to have a GUI where you can select a file and it will read in the data and let you know what the GPS date(s) is/are and allow you to set it to whatever it should be then write the binary file back out, either overwriting the file or writing a new one based on the user's choice.
Being from the USA I have an advantage over other bidders as we share the same timezones, so you don't have to wait overnight to get answers or receive feedback, and I'm a native english speaker so there won't be any communication problems.
Let me know how you'd like to proceed. I'm really looking forward to helping you out with this project.