Your method didn't compile in my Blitz.
It's probably because you didn't set up the .decls file. Create a plain text file with the extension name ".decls" (for example, "kernel32.decls") and put it in the blitz\userlibs folder. Then write these two lines:
.lib "Kernel32.dll" GetDateFormat%(iLocale%, iFlags%, iDate%, iFormat$, oDateBuf*, iSizeBuf%):"GetDateFormatA"
(I'm assuming you don't already have a '.lib "Kernel32.dll"' line anywhere else in any other decls file.)
Once you put in that GetDateFormat declaration underneath the .lib declaration, it should compile and run fine, assuming your Blitz version is up-to-date.