Workaround for missing std::chrono::from_stream

Question

C++20 introduces std::chrono::from_stream() to parse date/time from a string and store it in a std::chrono::time_point. For example:

std::stringstream ss("2018-12-09T00:00:00+0130");

std::chrono::time_point<std::chrono::utc_clock> tp;

if (std::chrono::from_stream(ss, "%FT%T%z", tp))
{
    std::cout << "Time point: " << tp << '\n';
}
else
    std::cout << "Not a valid timestamp\n";

However, GCC prior to version 14, as well as clang up to at least version 20.1, have not implemented this. Attempts to use from_stream result in a compiler error:

error: ‘from_stream’ is not a member of ‘std::chrono’

What workarounds are available in these versions? I can easily parse year/month/day/hour/minute/second/UTC offset (hours/minutes) into individual variables or a std::tm, but there doesn’t seem to be a straightforward way to construct a time_point from that.

system_clock has from_time_t() (utc_clock doesn’t), but that requires me to use legacy C code, which has its own flaws such as not being thread-safe.

How can I construct a std::chrono::time_point from pre-parsed field values, in a way that’s portable and doesn’t require new dependencies?

Theoratically yes, but I don’t want to introduce additional dependencies. I was looking for just a few lines of code. My use case should be much simpler than universal parsing, as I already have the numeric values of YYYY-MM-dd HH:mm:ss +/-HH:mm. No non-Gregorian calendars, no localized month or weekday names, no pretty timezone names and whatever else a full-blown parser would need to support. — user149408
– user149408, Commented Jun 14, 2025 at 12:32
Howard Hinnant is the original proposer of <chrono> library. His implementation is the reference for std. You won't get any better advice than the <date.h>. — Red.Wave
– Red.Wave, Commented Jun 14, 2025 at 13:01
@user149408 If you don't need the time zone database, you'll only need one header, date.h. When you later upgrade to a compiler with std::chrono::from_stream you can simply replace date:: with std::chrono::. — Ted Lyngmo
– Ted Lyngmo, Commented Jun 14, 2025 at 13:17

cpplearner · Accepted Answer · 2025-06-15 01:51:32Z

3

If you already have a struct tm with requested fields, you can convert it to a time_point of system_clock as follows:

constexpr std::chrono::sys_seconds from_tm(const std::tm& t)
{
    using namespace std::chrono;

    const auto y = static_cast<year>(t.tm_year + 1900);
    const auto m = static_cast<month>(t.tm_mon + 1);
    const auto d = static_cast<day>(t.tm_mday);
    const hours h{t.tm_hour};
    const minutes m{t.tm_min};
    const seconds s{t.tm_sec};

    sys_seconds res = sys_days{y/m/d};
    res = res + h + m + s;

    return res;
}

You can use std::chrono::clock_cast to convert the result to a time point of a different, compatible clock. For example:

std::chrono::time_point<std::chrono::utc_clock> tp
  = std::chrono::clock_cast<std::chrono::utc_clock>(res);

edited Jun 15, 2025 at 1:51

answered Jun 14, 2025 at 13:39

cpplearner

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1 Comment

user149408 Jun 14, 2025 at 14:53

It’s even easier when you have fields as individual variables as it saves you the conversion from the somewhat peculiar representation in std::tm. Also, UTC offsets need to be applied, i.e. subtracted. hh:mm:ss+zz:yy becomes hh-zz, mm-yy, ss – with a std::tm, you would have done that when populating it.

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