But the first parameter is described as usually a string, but this format being a POSSIBLE exception, using the term 'string' to refer to is doesn't seem that unnatural, and as I understand it, the value for that format could have been provided as a string too. The undefined behavior would also seem to be triggered if it wasn't a string in local time, so that would seem to apply if it wasn't a string. It also look like the documentation was fixed, and now refers to time values, not strings.