It took me several hours today to figure this one out, so I thought I’d write it up lest I forget.

I have a C++11 application that rapidly writes new documents to a MongoDB instance and I need high-resolution timestamps to determine the order things were written.

The problem is that the C++ standard time_t structure reports in seconds, so

builder.appendTimeT("updated_at",
		std::chrono::system_clock::to_time_t(std::chrono::system_clock::now())
);

creates an ISODate without milliseconds in MongoDB.

There are many solutions to be found using Boost::time¹, but since my code needs to be dependency-free and pure, I needed a C++11 clean solution.

MongoDB helps with its own Date_t class which is really just a long long number of milliseconds since the UNIX Epoch. So if I can generate that, we’d we dancing.

The solution in my utilities namespace, DateTime class is:

namespace utilities {

	struct DateTime {

		...

		static int64_t millisSinceEpoch()
		{
			std::chrono::system_clock::duration duration{
				std::chrono::system_clock::now().time_since_epoch()
			};
			return std::chrono::duration_cast<std::chrono::milliseconds>(duration).count();
		}

		...
	}
}  

This function constructs a C++11 duration object containing the number we need in an internal format, and then converts it to milliseconds for use.

The MongoDB c++ code also changes to:

builder.appendDate("updated_at", utilities::DateTime::millisSinceEpoch());

And now we have MongoDB ISODates with milliseconds that work perfectly.

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