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Yup, I agree the spec is clear on this specific point.  I'd rather 
the docs refer users to the platform C docs, though, as many 
other aspects of mktime can and do vary across 
implementations (for example, C is silent on the origin of the 
epoch, while POSIX defines it; C and POSIX are both clear as 
mud about how out-of-range struct tm members are to be 
treated; etc -- google for mktime man pages to see what a 
mess this function really is across platforms).