;don’t try to understand the code,
;it extensively uses black magic and Voodoo incantations

Source: Direct link to the source code
Software: asmutils
Programming language: Assembly

// calendar voodoo for computing the first day that would
// actually be displayed in the calendar, even if it’s from
// the previous month.
// WARNING: this is magic. ;-)

Source: Direct link to the source code
Software: DHTML Calendar
Programming language: JavaScript