Your Days Are Numbered in Calendar Math Book

This book is fun, but ultimately useless. You have to practise these tricks frequently in order to put them into long-term memory, and then when are you going to use them? Perhaps I should put that into the 1st person singular . . . I could learn these, and was having fun working through them all, but then thought about the likelihood of ever using them and decided that was about zero percent. I'm already good at simple mental arithmetic, good at estimating, and excellent with pen and paper, so for the limited amount of calculating that I need to do I have more than adequate skills already.
Still, a fun book for anyone who likes playing with numbers.