Convert std::chrono::milliseconds to ISO 8601 string - c++

Question

I've a std::chrono::milliseconds representing epoch unix time in milliseconds. I need to convert it into a string that follows the ISO 8601 format, like 2020-02-25T00:02:43.000Z.
Using date library I was able to parse it, with following GetMillisecondsFromISO5601String method:
#include "TimeConversion.hpp"
#include <date/date.h>
using std::string;
using std::string_view;
using std::chrono::milliseconds;
string TimeConversion::GetISO8601TimeStringFrom(const milliseconds& ms) {
std::stringstream ss;
date::to_stream(ss, "%FT%T%z", ms);
return ss.str();
}
milliseconds TimeConversion::GetMillisecondsFromISO5601String(string_view s) {
std::istringstream in{ std::move(string(s)) };
in.exceptions(std::ios::failbit);
date::sys_time<milliseconds> tp;
if (*s.rbegin() == 'z') {
in >> date::parse("%FT%T%z", tp);
}
else if (*s.rbegin() == 'Z') {
in >> date::parse("%FT%T%Z", tp);
}
else {
in >> date::parse("%FT%T%", tp);
}
return tp.time_since_epoch();
}
How can I do the inverse? I was trying to_stream in the GetISO8601TimeStringFrom method as you can see but without any results (it returns me an empty string).

Answer 1

The problem is that you're treating a time duration (milliseconds) as a time_point (time_point<system_clock, milliseconds>). All you need to do is convert the duration to time_point with explicit conversion syntax. The date lib has a convenience type alias for this type: sys_time<milliseconds>:
string TimeConversion::GetISO8601TimeStringFrom(const milliseconds& ms) {
date::sys_time<milliseconds> tp{ms};
// continue using tp ...
You may also use the more convenient format function in place of to_stream:
string TimeConversion::GetISO8601TimeStringFrom(const milliseconds& ms) {
return date::format("%FT%T%z", date::sys_time<milliseconds>{ms});
}