What is the best approach to get a millisecond-rounded timestamp string in Howard Hinnant's Date library?

Question

I have the following code, using Date library:

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

using namespace date;
using namespace std::chrono;

int main()
{
    auto now = system_clock::now();
    std::stringstream ss;
    ss << now;
    std::string nowStr = ss.str();   // I need a string
    std::cout << nowStr << " UTC\n";
}

The result is:

2020-03-26 17:38:24.473372486 UTC

Is stringstream the correct approach to obtain a string from the chrono::timepoint that now() returns? And, if so, how can I round those nanoseconds to milliseconds?

Answer 1

Yes, ostringstream is a good way to do this. You could also use date::format which returns a string, but this still uses a ostringstream internally:

string s = format("%F %T %Z", now);

With either technique, you can truncate to milliseconds output by truncating the input time_point to milliseconds prior to formatting it. You can choose any of these rounding modes:

• round towards the epoch date: time_point_cast<milliseconds>(now)
• round towards the past : floor<milliseconds>(now)
• round towards the future : ceil<milliseconds>(now)
• round towards the nearest (towards even on tie): round<milliseconds>(now)

-

string s = format("%F %T %Z", floor<milliseconds>(now));

2020-03-26 17:38:24.473 UTC

In C++20 this will become:

string s = format("{:%F %T %Z}", floor<milliseconds>(now));