Pre-Millennium Tension: The Dali Clock Y2K Easter Egg
jwz.org
jwz.org
Edit: Based on the version file inside the package marked 1998 in that index, it was last modified at the very end of 1997, so it won't contain the prank code if jwz's dates are right. The one immediately above it, however, despite being listed as a 2001 release, has a modification date of November, 1999.
xdaliclock-2.18/X11/digital.c starting at line 1260. He just uses mktime() with a year of 100.