How to convert std::chrono::system_clock::now() to boost::mysql::datetime::time_point

Question

I'm using the boost mysql library in a project. I need to use boost::mysql::datetime (datetime) to represent timestamps. A datetime value can be constructed from a boost::mysql::datetime::time_point.

using time_point = std::chrono::time_point< std::chrono::system_clock, std::chrono::duration< std::int64_t, std::micro >>;

How do I convert a "normal" std::chrono::system_clock::time_point to a boost::mysql::datetime::time_point?

As an interesting side note, boost::mysql does not use the boost Gregorian date library.

Details:

Ubuntu 24.04
C++23
Boost 1.88
gcc-14

Code:

#include <boost/asio.hpp>
#include <boost/mysql.hpp>
#include <chrono>
#include <exception>
#include <format>
#include <iostream>
#include <stdexcept>

int main()
{
    boost::mysql::datetime::time_point tp = std::chrono::system_clock::now();
    const std::time_t t_c = std::chrono::system_clock::to_time_t(tp);
    std::cout << "The system clock is currently at " << std::ctime(&t_c);
    
    return EXIT_SUCCESS;
}

The error for the above code:

main.cpp:11:75: error: conversion from ‘time_point<[...],duration<[...],ratio<[...],1000000000>>>’ to non-scalar type ‘time_point<[...],duration<[...],ratio<[...],1000000>>>’ requested
11 | boost::mysql::datetime::time_point tp = std::chrono::system_clock::now();

In case you want to reproduce my error message, here is the CMakeLists.txt file:

cmake_minimum_required(VERSION 3.31)

project(testTime LANGUAGES CXX)

if(POLICY CMP0167)
    cmake_policy(SET CMP0167 NEW)
endif()

find_package(Boost 1.87.0 REQUIRED COMPONENTS system charconv program_options)

add_executable(testTime main.cpp)

target_compile_options(testTime PRIVATE -Wall -Wextra -pedantic -Werror)

target_compile_features(testTime PRIVATE cxx_std_23)

target_link_libraries(testTime  ${Boost_LIBRARIES} ssl crypto)

@ChristianStieber I tried std::chrono::clock_cast, not time_point_cast. — pacmaninbw
– pacmaninbw, Commented Aug 31 at 14:48
Also note that mysql::datetime::now() does exactly this. You might find it useful. Should you need any more help, feel free to open a GitHub issue, since I tend to see these faster (I'm the author) — anarthal
– anarthal, Commented Sep 7 at 15:34
@anarthal Thank you for the heads up. I was using this as an example for a minimum reproducible case. My code stores std::chrono values in classes that are used throughout the application and then stores the classes in the database. Example, the last time a user would log into the system would be stored as a std::chrono::time_point (system clock). GitHub repository. You will find the Boost::MySql usage code in a side directory. — pacmaninbw
– pacmaninbw, Commented Sep 7 at 16:07

Howard Hinnant · Accepted Answer · 2025-09-03 01:10:03Z

10

The chrono system does not allow implicit conversion from fine precision to coarser (though the reverse is allowed implicitly).

On your platform system_clock::now() returns nanoseconds precision and you desire microseconds precision.

There are 4 named conversion functions to do this, which differ only in rounding mode (how to truncate the extra nanoseconds):

time_point_cast: truncates towards the epoch of 1970-01-01 00:00:00 UTC.
floor: truncates to the previous microsecond.
ceil : truncates to the next microsecond.
round : truncates to the nearest microsecond.

Example:

namespace dt = std::chrono;
auto tp = dt::floor<dt::microseconds>(dt::system_clock::now());
std::cout << "The system clock is currently at " << tp << '\n';

Note: time_point_cast is equivalent to floor for times after the epoch, and equivalent to ceil for times prior to the epoch.

Note: If the source time happens to be on a microsecond boundary, then all 4 rounding modes give the same result (the microsecond the time_point is on).

edited Sep 3 at 1:10

answered Aug 31 at 15:06

Howard Hinnant

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2 Comments

Jacobi John Sep 3 at 0:51

given there's a cmath function called ceil and floor, it would be easier to read/learn from your example if you used the complete namespace in the call like: std::chrono::floor<std::chrono::microseconds>(system_clock::now()); i know it's longer, but it removes all doubt.

Howard Hinnant Sep 3 at 1:11

Good comment. I'm allergic to excessive verbosity. I've struck a compromise with a namespace alias.

Alan Birtles · Accepted Answer · 2025-08-31 14:58:49Z

3

You need to use std::chrono::time_point_cast to convert to a time_point with a different duration:

boost::mysql::datetime::time_point boost = std::chrono::time_point_cast<boost::mysql::datetime::time_point::duration>(std::chrono::system_clock::now());

answered Aug 31 at 14:58

Alan Birtles

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1 Comment

pacmaninbw Aug 31 at 15:24

Good answer, and it works! I am going to accept the other answer because it is more complete.

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