CODEWITHSUNDEEP
c++c++-chronofmt
Rob N
Rob N

Reputation: 16409

How to print the current time with fractional seconds, using fmt library

This code

#include <chrono>
#include <fmt/format.h>
#include <fmt/chrono.h>
...
auto now = std::chrono::system_clock::now();
fmt::print("The time is: {:%Y-%m-%d %H:%M:%S}\n", now);

Prints this:

The time is: 2023-01-02 15:51:23

How do I get it to print sub-second precision, for example, milliseconds? Something like:

The time is: 2023-01-02 15:51:23.753

Upvotes: 4

Views: 1984

Answers (2)

zerox
zerox

Reputation: 397

Just share my way to output fractional seconds in logging:

std::string Timestamp() {
    namespace c = std::chrono;
    auto tp = c::time_point_cast<c::microseconds>(c::system_clock::now());
    auto us = tp.time_since_epoch().count() % std::micro::den;
    return fmt::format("{:%Y-%m-%d_%H:%M:%S}.{:06d}", tp, us);
}

There is a pull request to output fractional seconds for formatting time_point, (but closed, the author did not continue the work):

https://github.com/fmtlib/fmt/pull/2292

My concern is that change %S to include fractional part is some-what breaking change (strftime defines it as seconds from 00 to 60). I hope fmtlib would consider some other way to tackle this requirement.

Upvotes: 2

Dirk is no longer here
Dirk is no longer here

Reputation: 368261

I think you have to do this in two steps -- YMD and HM followed by the high-res seconds. Here is a worked example, using fmt version 9.0 on Ubuntu:

#include <chrono>
#include <fmt/format.h>
#include <fmt/chrono.h>

int main() {
    auto now = std::chrono::system_clock::now();
    auto sse = now.time_since_epoch();
    fmt::print("{:%FT%H:%M:}{:%S}\n", now, sse);
    exit(0);
}

for which I get

$ g++ -o answer answer.cpp -lfmt; ./answer 
2023-01-02T16:26:17.009359309
$

(Initial, earlier, slightly off attempt to reason based on spdlog follows. But spdlog wraps fmt and overlays its format.)

It is on the format page, you want something like %Y-%m-%d %H:%M:%S.%e. Note that %e, %f and %F give you milli, micro and nanoseconds. Edit: That was the spdlog format though.

Here is a quick example, for simplicity from my R package RcppSpdlog accessing fmt via spdlog:

> library(RcppSpdlog)
> RcppSpdlog::log_set_pattern("[%Y-%m-%d %H:%M:%S.%f] [%L] %v")
> RcppSpdlog::log_warn("Hello")
[2023-01-02 15:00:49.746031] [W] Hello
> RcppSpdlog::log_set_pattern("[%Y-%m-%d %H:%M:%S.%e] [%L] %v")
> RcppSpdlog::log_warn("World")
[2023-01-02 15:01:02.137] [W] World
>

Note the display of micro- and milliseconds as stated.

Edit: There is something else going on with your example. Here is a simplified answer, printing only seconds and it comes by default fractionally down to nanoseconds (for me on Linux):

$ cat answer.cpp
#include <chrono>
#include <fmt/format.h>
#include <fmt/chrono.h>

int main() {
    fmt::print("{:%S}\n", std::chrono::system_clock::now().time_since_epoch());
    exit(0);
}
$ g++ -o answer answer.cpp -lfmt
$ ./answer 
02.461241690
$

Upvotes: 7

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