Drawbacks of C++20 std::chrono::duration formatting

Question

I needed to print std::chrono::duration that may be greater than 24 hours in easily-readable way, like "2 d. 09:10:11".

C++20's std::chrono::time_point and std::chrono::duration formatting (based on Howard Hinnant's date library) contains tons of useful format specifiers but does not have one for number of days in std::chrono::duration. The closest one is %j but it pads it with zeros. Also there is no specifier for number of hours within the day. The closest are %H (but it includes whole days hours too) and %OH (but it crashes on assertion that number of hours is less than 24).

Am I missing something?

Does it make sense to add more specifiers to C++20's std::format?

So far I'm using workaround function:

#include <chrono>
#include <cstring> // std::strncmp
#include <date.h> // use Howard Hinnant's date library until C++20 is supported by compilers
#include <string>

// Workaround for C++20 std::chrono::format's inability to produce number of days in std::chrono::duration.
// Temporary format specifier for this is `%J`. It must be in the beginning of the format string.
template<typename Rep, typename Period>
std::string duration_to_str(const char* format, const std::chrono::duration<Rep, Period>& duration)
{
    if (std::strncmp(format, "%J", 2))
        return date::format(format, duration);
    static const auto one_day = std::chrono::hours(24);
    const size_t days = std::chrono::duration_cast<std::chrono::hours>(duration).count() / 24;
    return std::to_string(days) + date::format(format + 2, duration - days * one_day);
}

Test:

#include <iostream>

template<typename Rep, typename Period>
void test_dur_to_str(const std::chrono::duration<Rep, Period>& duration)
{
    std::cout << duration_to_str("%T", duration) << ":\n"
              << duration_to_str("%j d. %T", duration) << '\n'
              << duration_to_str("%J d. %T", duration) << "\n\n";
}

int main()
{
    test_dur_to_str(std::chrono::seconds(24 * 60 * 60 - 1));
    test_dur_to_str(std::chrono::seconds(24 * 60 * 60));
    test_dur_to_str(std::chrono::seconds(91297));
    test_dur_to_str(std::chrono::seconds(2 * 24 * 60 * 60 - 1));
    test_dur_to_str(std::chrono::seconds(2 * 24 * 60 * 60));
    test_dur_to_str(std::chrono::seconds(2 * 24 * 60 * 60 + 1));
    test_dur_to_str(std::chrono::seconds(13 * 24 * 60 * 60 + 5));
    test_dur_to_str(std::chrono::seconds(365 * 24 * 60 * 60 + 5));
    test_dur_to_str(std::chrono::seconds(366 * 24 * 60 * 60 + 5));
    test_dur_to_str(std::chrono::seconds(367 * 24 * 60 * 60 + 5));
}

Output:

23:59:59:
000 d. 23:59:59
0 d. 23:59:59

24:00:00:
001 d. 24:00:00
1 d. 00:00:00

25:21:37:
001 d. 25:21:37
1 d. 01:21:37

47:59:59:
001 d. 47:59:59
1 d. 23:59:59

48:00:00:
002 d. 48:00:00
2 d. 00:00:00

48:00:01:
002 d. 48:00:01
2 d. 00:00:01

312:00:05:
013 d. 312:00:05
13 d. 00:00:05

8760:00:05:
365 d. 8760:00:05
365 d. 00:00:05

8784:00:05:
366 d. 8784:00:05
366 d. 00:00:05

8808:00:05:
367 d. 8808:00:05
367 d. 00:00:05

Answer 1

I want to show simplified version of workaround function, as suggested by Howard Hinnant in his comment, for those who may find it useful:

#include <chrono>
#include <cstring> // std::strncmp
#include <date.h> // use Howard Hinnant's date library until C++20 is supported by compilers
#include <string>

template<typename Rep, typename Period>
std::string duration_to_str(const char* format, const std::chrono::duration<Rep, Period>& duration)
{
    if (std::strncmp(format, "%J", 2))
        return date::format(format, duration);
    const auto days = std::chrono::duration_cast<date::days>(duration);
    return std::to_string(days.count()) + date::format(format + 2, duration - days);
}