The standards for mktime() define a return value of (time_t)-1 as indicating that the time specified cannot be represented. However, a return value of -1 is also a valid time_t corresponding to 31-DEC-1969 23:59:59 GMT. How do we distinguish between the two?
Call localtime() on the seemingly-in-error mktime() result, and compare the relevant portions of the structs tm. A match implies a non-error.
Since there's normalization going on in mktime, the tm that comes back from localtime may not match the one that went in to mktime even if -1 is being returned as a legitimate time_t rather than an error indication. Another possibility is to save away the original tm structure; if mktime returns -1, bump the tm_sec value by 1, call mktime again, and see if it returns zero. --ado