CODEWITHSUNDEEP
c++datetimec++11c++-chrono
Emile Cormier
Emile Cormier

Reputation: 29209

How do I convert YYYY/MM/DD HH:MM:SS to std::chrono::system_clock::time_point?

I'm obtaining YYYY/MM/DD HH:MM:SS components from an offboard real time clock chip. I want to convert this to a std::chrono::system_clock::timepoint so that I may obtain the seconds since the Epoch and update the operating system's time.

I would like to use Howard Hinnant's proposed date library to perform this conversion.

I would intuitively do something like this:

date::year_month_day ymd = ...;
date::time_of_day tod = ...;
auto sysTime = date + tod;

but I don't see an appropriate operator+ overload for this. Am I missing something?

Another use case for this type of conversion is to convert a calendar date and time into a std::chrono::timepoint that I can pass to boost::asio::steady_timer.

Upvotes: 1

Views: 2461

Answers (1)

Howard Hinnant
Howard Hinnant

Reputation: 218740

Typically a time_of_day is just used for formatting a duration into hh:mm:ss.fff for output. But it is possible to turn a time_of_day back into a std::chrono::duration:

auto tod = make_time(hours{17} + minutes{16} + seconds{45});  // time_of_day
auto d = seconds(tod);                                        // seconds

In C++14 I would’ve used the more concise: 17h + 16min + 45s, but as you mentioned you were in C++11, I’ll stick to C++11 code.

A year_month_day can be converted into a sys_days with:

auto ymd = jul/29/2015;
auto dp = sys_days{ymd};

And a sys_days is nothing but a system_clock::time_point but with a resolution of days instead of microseconds or nanoseconds.

Then you can add a sys_days and duration and assign that to a system_clock::time_point. Putting it all together:

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

int
main()
{
   using namespace date;
   using namespace std::chrono;
   auto ymd = jul/29/2015;
   auto tod = make_time(hours{17} + minutes{16} + seconds{45});
   system_clock::time_point tp =  sys_days(ymd) + seconds(tod);
}

In this simplified example there is no need to go through time_of_day, and you could just stay with durations:

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

int
main()
{
   using namespace date;
   using namespace std::chrono;
   auto ymd = jul/29/2015;
   system_clock::time_point tp = sys_days(ymd)
                               + hours{17} + minutes{16} + seconds{45};
}

The main thing to remember is that sys_days is just a coarse system_clock::time_point, and when you convert to that from a year_month_day, then you are completely in the std::chrono system and can do anything you want with time_points and durations from there.

Upvotes: 2

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