I think it's a bug in the unzip utility. Slackware has (prematurely?) enabled the -DIZ_HAVE_UXUIDGID option when compiling the package and this causes the breakage. The option does not show up on "unzip -v" either.

In unzip60 sources, process.c:2909

        flags &= ~0x0ff;      /* ignore any previous UNIX field */
[download]

That results in dos time fields being used instead of unix. I have no idea if there's a bugreport or a fix available somewhere.

Just my 10 min debug. HtH.