What are Lilian Days?

If you've ever used the CEEDATE() or CEEDAYS() API's on the iSeries you know that they both reference a parameter called "Lilian Days". In case you're curious, it's named after Aloisius Lilius who was an adviser to Pope Gregory XIII and an important developer of the Gregorian Calendar.

What I found most interesting (in the geeky sense) is that the concept of Lilian Days was invented by an Ib'm high priest named Bruce G. Ohms when his article "Computer Processing of Dates Outside the Twentieth Century" was published in the Ib'm Systems Journal, Volume 25, in 1986.