strptime() [at least] falls into this category -- I believed Guy intended it as the inverse of strptime() [or at least as close as you can get].

"Inverse of 'strftime()'". Yes, that was the basic idea. It does take a format parameter; however, it hasn't any magic way of figuring out whether 1/2/92 is January 2, 1992 or February 1, 1992 - you have to tell it how to interpret dates such as that, by passing it the right format string.