Poco DateTimeFormatter - Print timestamp with current timezone - c++

Question

How do I print a Poco::Timestamp with Poco::DateTimeFormatter into a formatted date-time based on the current timezone?
I have a print_pretty_datetime(const Poco::Timestamp &now) where I'll receive a Poco::Timestamp so I can't use a Poco::LocalDateTime unfortunately.
MCVE:
#include "Poco/Timestamp.h"
#include "Poco/Timezone.h"
#include "Poco/DateTimeFormatter.h"
#include "Poco/DateTimeFormat.h"
#include <iostream>
#include <string>
// Cannot change the method signature. I will receive a Poco::Timestamp object
std::string print_pretty_datetime(const Poco::Timestamp &now)
{
return Poco::DateTimeFormatter::format(
now,
Poco::DateTimeFormat::SORTABLE_FORMAT,
Poco::Timezone::tzd()
);
}
int main()
{
Poco::Timestamp now;
std::string dt_now = print_pretty_datetime(now);
std::cout << dt_now << '\n';
return 0;
}
E.g: the string returned is 2019-01-07 11:10:12 (thus UTC+0) whereas my device is in in UTC+1.
In fact, the command date returns Mon Jan 7 12.10.12 CET 2019.
What is the correct parameter for tzd in Poco::DateTimeFormatter::format for printing the date-time based on current locale?
System info:
SMP Debian 4.9.130-2 (2018-10-27) x86_64 GNU/Linux
g++ (Debian 6.3.0-18+deb9u1) 6.3.0 20170516
Poco 1.9.0

Answer 1

Get local time by Poco::LocalDateTime, then read its timestemp and pass it into formatter:
Poco::LocalDateTime dateTime;
Poco::Timestamp now = dateTime.timestamp();
You need to define your own string format which includes information about time-zone, because SORTABLE_FORMAT doesn't handle it.
Hint (you can open source of DateTimeFormatter and see how timeZoneDifferential is processed in append function - this parameter doesn't affect hours in output).
So if you change to:
std::string dt_now = Poco::DateTimeFormatter::format(
now,
"%H:%M:%S %z,%Z",
Poco::Timezone::tzd()
);
you will see HH::MM::SS +01:00,+0100 in output.

Answer 2

You can create a Poco::LocalDateTime in the system's current timezone from a Poco::Timestamp via an intermediate Poco::DateTime object.
#include "Poco/LocalDateTime.h"
#include "Poco/DateTime.h"
#include "Poco/DateTimeFormat.h"
#include "Poco/DateTimeFormatter.h"
#include <iostream>
int main(int argc, char** argv)
{
Poco::Timestamp ts;
Poco::DateTime dt(ts);
Poco::LocalDateTime ldt(dt);
std::string str = Poco::DateTimeFormatter::format(ldt, Poco::DateTimeFormat::SORTABLE_FORMAT);
std::cout << str << std::endl;
return 0;
}

Answer 3

Alright, if you don't necessarily need to use a Poco::Timestamp, a solution can be simply to use a Poco::LocalDateTime:
#include "Poco/LocalDateTime.h"
#include "Poco/DateTimeFormatter.h"
#include "Poco/DateTimeFormat.h"
#include <iostream>
#include <string>
int main()
{
Poco::LocalDateTime now;
std::string dt_now = Poco::DateTimeFormatter::format(
now,
Poco::DateTimeFormat::SORTABLE_FORMAT
);
std::cout << dt_now << '\n';
return 0;
}
and this will print the date based on the current timezone.
I still have to use the Poco::Timestamp though as I'll get it as parameter to my print_pretty_datetime(const pc::Timestamp &now), so this answer does not yet applies to my case.