...

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.

This still doesn't tell me whether the -1 return from mktime() meant that the time was not representable. It does tell me whether it is representable one second later. If a particular implementation of mktime() doesn't support times before the Epoch (and that seems okay as far as the standards go), then a return of 0 resulting from bumping the tm_sec up by 1 would lead me to the wrong conclusion concerning my original call.

Oops...point well taken! Would subtracting one from tm_sec (and looking for a return of -2) work? --ado