how do I parse an iso 8601 date (with optional milliseconds) to a struct tm in C++?

New answer for old question. Rationale: updated tools.

Using this free, open source library, one can parse into a std::chrono::time_point<system_clock, milliseconds>, which has the advantage over a tm of being able to hold millisecond precision. And if you really need to, you can continue on to the C API via system_clock::to_time_t (losing the milliseconds along the way).

#include "date.h"
#include <iostream>
#include <sstream>

date::sys_time<std::chrono::milliseconds>
parse8601(std::istream&& is)
{
    std::string save;
    is >> save;
    std::istringstream in{save};
    date::sys_time<std::chrono::milliseconds> tp;
    in >> date::parse("%FT%TZ", tp);
    if (in.fail())
    {
        in.clear();
        in.exceptions(std::ios::failbit);
        in.str(save);
        in >> date::parse("%FT%T%Ez", tp);
    }
    return tp;
}

int
main()
{
    using namespace date;
    using namespace std;
    cout << parse8601(istringstream{"2014-11-12T19:12:14.505Z"}) << '\n';
    cout << parse8601(istringstream{"2014-11-12T12:12:14.505-5:00"}) << '\n';
}

This outputs:

2014-11-12 19:12:14.505
2014-11-12 17:12:14.505

Note that both outputs are UTC. The parse converted the local time to UTC using the -5:00 offset. If you actually want local time, there is also a way to parse into a type called date::local_time<milliseconds> which would then parse but ignore the offset. One can even parse the offset into a chrono::minutes if desired (using a parse overload taking minutes&).

The precision of the parse is controlled by the precision of the chrono::time_point you pass in, instead of by flags in the format string. And the offset can either be of the style +/-hhmm with %z, or +/-[h]h:mm with %Ez.