TurboTax

TurboTax is a series of tax preparation programs by Intuit for the U.S. federal and state income tax.

The government ensures continued employment for the producers of tax programs by changing the laws, regulations, and forms every year, necessitating corresponding changes to all the software. As a result, programs such as TurboTax use a different file format every year to correspond to the tax forms the data is intended to fill out. This results in a format nightmare.

Formats for TurboTax program (version you install on your computer)
For a while, TurboTax used the  extension for its data, but eventually switched to a year-specific extension like. Each year's program will only directly open that year's data, though they can import data from past years' returns where this is useful for carrying things over. (To this end, the program will generally recognize the last few years' formats for TurboTax, and even competing programs such as that of H&R Block.)

The format appears to be a proprietary binary format, with the first four bytes spelling  in ASCII (54 54 46 4E in hexadecimal), apparently the "signature bytes" indicating that a file is of a TurboTax format. (Ta Ta For Now? No, it probably stands for something like Turbo Tax Financial Network.)

Format for online version
Lately they are heavily promoting an online version of TurboTax which you complete through their secure website. There, the calculations are done "in the cloud", and your data is stored there, but at the end of the process you get a chance to download your return. The main downloads are in PDF form, showing the various tax forms and worksheets, but there is also a feature to download a file with a  extension that apparently contains the return in a format that can be imported back into TurboTax. This seems to be encoded binary data of some sort. One file of this sort has  in ASCII as its opening four bytes, but without further examples it is not possible to tell if this is true of all such files. The 2014 version still says that it's saving a ".tax file", but the actual file produced has a  extension and starts with   like the desktop-version files.

Other notes
There does not appear to be any documentation of the specifics of the file format online, though the fact that competing tax programs are able to import past returns in the format implies that their makers somehow managed to either obtain or reverse-engineer the format.

TurboTax also uses the TXF tax interchange format to import data from other financial programs and websites.

Links

 * Official site
 * Official documentation on finding and opening earlier years' tax files (opening them requires obtaining the earlier TurboTax software)