How to find difference in time in seconds and nanoseconds between two dates?

Question

I need to calculate the seconds and nanoseconds between the Unix Epoch (00:00:00 January 1st 1970 UTC) and some date in the future the user will select.

Here is what I managed to find so far looking through other Stack Overflow answers:

// seconds and nanoseconds past epoch
auto now = std::chrono::system_clock::now().time_since_epoch();
auto now_ns = std::chrono::duration_cast<std::chrono::nanoseconds>(now);
auto now_sec = std::chrono::duration_cast<std::chrono::seconds>(now);

// output total ns since epoch
qDebug() << now_ns.count() << "ns";
// output total secs since epoch
qDebug() << now_sec.count() << "secs";

auto nano_secs = now_ns.count() - (now_sec.count() * 1000000000);
// output ns minus whole seconds
qDebug() << "nano_secs = " << nano_secs;

Using this, I am able to get the result I am looking for for the current date and time but that's not what I need. Let's say for demonstration purposes that future date is 00:00:00 July 1st 2020 UTC. How can I go about calculating this?

`(now_sec.count() * 1000000000)`dont do this - use duration_cast and then do the subtraciton. — Mike Vine, Jun 09 '20 at 19:33
`auto nano_secs = now_ns - now_sec;` looks reasonable to me (no `duration_cast` at all). — Howard Hinnant, Jun 09 '20 at 19:37

score 0 · Accepted Answer · answered Jun 10 '20 at 00:11

In C++20 this looks like:

using namespace std::chrono;
nanoseconds now_ns = sys_days{July/1/2020}.time_since_epoch();
auto now_sec = duration_cast<seconds>(now_ns);
now_ns -= now_sec;
std::cout << now_sec << ", " << now_ns << '\n';

Output:

1593561600s, 0ns

Finding a C++20 std::lib that can do this today is going to be difficult. However here is a free, open-source preview of C++20 <chrono>.

Just add #include "date/date.h" and using namespace date;, and it works.

If you don't want to use this 3rd party header, here is the algorithm to convert a {y, m, d} triple into a count of days since the Unix epoch. Indeed, all sys_days{July/1/2020} does is call days_from_civil under the hood and put it into a days-precision chrono::time_point based on system_clock. From then on the C++11/14 version of <chrono> does all of the work.

None of the above computations are dependent on your computer's local timezone setting.

How to find difference in time in seconds and nanoseconds between two dates?

1 Answers1