Some comments from @sipa in #12702.

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Yes, I think `importmulti` is superior in every way, except:
* Slightly less convenient to use (it requires redundant information, as you have to explicitly state what you want to import, and then give the keys/... to enable that).
* ~Doesn't support P2SH-P2WPKH yet~. Added in #14454

The latter can be fixed by just adding support for that case. The former can be improved by adding 'default' cases (e.g. if you prove a WIF private key, it can permit omitting the address, ...).

static const boost::posix_time::ptime epoch = boost::posix_time::from_time_t(0);
    static const std::locale loc(std::locale::classic(),
        new boost::posix_time::time_input_facet("%Y-%m-%dT%H:%M:%SZ"));
    std::istringstream iss(str);
    iss.imbue(loc);
    boost::posix_time::ptime ptime(boost::date_time::not_a_date_time);
    iss >> ptime;
    if (ptime.is_not_a_date_time())
        return 0;
    return (ptime - epoch).total_seconds();
}
std::string static EncodeDumpString(const std::string &str) {