how can i make UTC date time compatible with postgres TIME from a nanosecond timestamp ?

[JourneyToSilius]

    std::chrono::milliseconds milliTime = std::chrono::milliseconds(message.E);

    std::chrono::nanoseconds ns = std::chrono::duration_cast<std::chrono::nanoseconds> (milliTime);

    int64_t nanoTime = ns.count();

So I've got a nano timestamp as int64_t and I need to make a date time UTC compatible with postgres TIME format UTC

I have been trying to find answers in stack overflow but none related on how to do this specific parsing

I am trying to do this : date::sys_time<std::chrono::nanoseconds>{nanoTime};

but it does not accept the this type for conversion ...

Thanks !

[HowardHinnant]

Sorry, I'm not familiar with postgres. However, the input seems to be an integral number of milliseconds contained in message.E. And I'm going to guess that this number of milliseconds is a time duration since the Unix Time epoch of 1970-01-01. Is that right?

The output I'm more confused about: Is it a date/time string you're looking for? Or a count of nanoseconds since the Unix Time epoch?

[HowardHinnant]

date::sys_time<std::chrono::nanoseconds>{ns}; will work. The input needs to have type nanoseconds, or something implicitly convertible to nanoseconds such as:

date::sys_time<std::chrono::nanoseconds>{std::chrono::milliseconds{message.E}};

[JourneyToSilius]

Hi Howard and thank you for your answer !

The format in postgres I'm using is this, which is UTC time witth nanosecond precission : 2022-07-10 01:03:57.369263+0

Thanks I'm going to try this right now

[JourneyToSilius]

ok now it compiles, thanks ! How can I convert it to a string ?

[HowardHinnant]

Before I answer, clarification: You're asking for nanosecond precision, but the example you show has microsecond precision (6 places vs 9). Which should it be?

[JourneyToSilius]

ah yes, sorry ... this was another database example I was looking at. It's on nano, yes ! Thank you

[HowardHinnant]

#include "date/date.h"
#include <chrono>
#include <iostream>

int
main()
{
    using namespace date;
    using namespace std;
    using namespace std::chrono;

    int64_t input = 1657415037369263000;  // nanoseconds of Unix Time
    sys_time<nanoseconds> tp{nanoseconds{input}};
    auto str_out = format("%F %T+0", tp);
    cout << str_out << '\n';
}

Output:

2022-07-10 01:03:57.369263000+0

[JourneyToSilius]

thank you very much, I got it working now. Just one more thing, is there a section where I can look at all the options for time formatiing ? just in case since I will use this extensively and I might need to do other resolutions, like microseconds, miliseconds, etc

Thanks !

[HowardHinnant]

Yes, here are all of the formatting options: https://howardhinnant.github.io/date/date.html#to_stream_formatting

[HowardHinnant]

To change the resolution, truncate the time stamp passed to format using one of time_point_cast, floor, ceil, or round. E.g.:

    auto str_out = format("%F %T+0", floor<milliseconds>(tp));

[JourneyToSilius]

I'm studying it now. thank you for your prompt answers, I will close this now :)

[JourneyToSilius]

Hi Howard

I am struggling with the format on postgres, it doesn't accept the one date.h produces

I had a POC of this program in Go, which worked out of the box with TIMESTAMPZ postgres format. The Go produced string was this : 2022-08-02 16:37:20.472778873 +0000 UTC

the one date.h is producing right now is: 2022-08-02 16:37:20.472778873+0

[HowardHinnant]

Try:

auto str_out = format("%F %T %z %Z", tp);