I recently ended up in a wikipedia rathole looking at various alternative calendar systems, at least in part to be able to give a true but unintelligible answer to the question of when my birthday is. Among the oddballs were the Discordian calendar which divides the year into five seasons of 73 days each with 5-day weeks and the Pataphysical calendar which has 13 months each of 29 days, but for most months, the 29th day is imaginary.