How does one print an iso timestamp in a smart contract

eosio.cdt doesn't support time library in its libc, so you cannot use time related functions. eoscc, a customized version of eosio.cdt, supports it. (I'm a writer of that)

You can check it from here.

• Use std::strftime instead of std::put_time. put_time needs stream.
• std::time_t accepts seconds, so do not multiply time_point::sec_since_epoch() with 1000.
• Use std::gmtime instead of std::localtime. localtime requires timezone.
• In contract runtime, timezone info is not available, so you had better use the format "%FT%T" (or "%FT%TZ" for indicating UTC).

char buffer[32];
time_t current_time = current_time_point().sec_since_epoch();
std::strftime(buffer, sizeof(buffer), "%FT%TZ", std::gmtime(&current_time));
print(buffer);