Time Zones: Online Classes

Today, I will continue to present the functionality of the time zones of the C++20 chrono extension.

This post is the eight in my detailed journey through the chrono extension in C++20:

Various Time Zones for Online Classes

The program onlineClass.cpp answers the following question: How late is it in given time zones when I start an online class at the 7h, 13h, or 17h local time (Germany)?

The online class should start on February 1st, 2021, and last 4 hours. Because of daylight savings time, the calendar date is essential to get the correct answer.

#include <chrono>
#include <algorithm>
#include <iomanip>
#include <iostream>

using namespace std::chrono_literals;

template <typename ZonedTime>
auto getMinutes(const ZonedTime& zonedTime) {                               // (1)              
    return std::chrono::floor<std::chrono::minutes>(zonedTime.get_local_time());

void printStartEndTimes(const std::chrono::local_days& localDay,           // (2) 
                        const std::chrono::hours& h, 
                        const std::chrono::hours& durationClass,
                        const std::initializer_list<std::string>& timeZones ) {
    std::chrono::zoned_time startDate{std::chrono::current_zone(), localDay + h}; // (4)  
    std::chrono::zoned_time endDate{std::chrono::current_zone(), 
                                    localDay + h + durationClass};
    std::cout << std::format("Local time: [{}, {}]\n", getMinutes(startDate), getMinutes(endDate));
    auto longestStringSize = std::max(timeZones, [](const std::string& a,  // (5) 
                       const std::string& b) { return a.size() < b.size(); }).size();

    std::string formatTimeZones = "  {0:<{3}} [{1}, {2}]\n";              // (7) 
    for (auto timeZone: timeZones) {                                      // (6) 
        std::cout << std::vformat(formatTimeZones, std::make_format_args(timeZone, 
                                                            getMinutes(std::chrono::zoned_time(timeZone, startDate)),
                                                            getMinutes(std::chrono::zoned_time(timeZone, endDate)),
                                                            longestStringSize + 1));                             

int main() {

    using namespace std::string_literals;
    std::cout << '\n';

    constexpr auto classDay{std::chrono::year(2021)/2/1};
    constexpr auto durationClass = 4h;
    auto timeZones = {"America/Los_Angeles"s, "America/Denver"s, 
                      "America/New_York"s, "Europe/London"s, 
                      "Europe/Minsk"s, "Europe/Moscow"s, 
                      "Asia/Kolkata"s, "Asia/Novosibirsk"s, 
                      "Asia/Singapore"s, "Australia/Perth"s, 

    for (auto startTime: {7h, 13h, 17h}) {                              // (3) 
        printStartEndTimes(std::chrono::local_days{classDay}, startTime, 
                           durationClass, timeZones);
        std::cout << '\n';

    std::cout << '\n';


Before I dive into the functions getMinutes (line 1) and printStartEndTimes (line 2), let me say a few words about the main function. The main function defines the day of the class, the duration of the class, and all time zones. Finally, the range-based for-loop (line 3) iterates through all potential starting points for an online class. All necessary information is displayed thanks to the function
(line 2).

The few lines beginning with line (4) calculate the startDate and endDate of my training by adding the start and class duration to the calendar date. Both values are displayed with the help of the function getMinutes (line 1). std::floor<std::chrono::minutes>(zonedTime.get_local_time()) gets the stored time point out of the std::chrono::zoned_time and truncates the value to the minute resolution. To properly align the output of the program, line (5) determines the size of the longest of all time-zone names. Line (6) iterates through all time zones and displays the name of the time zone and the beginning and end of each online class. A few calendar dates even cross the day boundaries.

The format string in line 7 ” {0:<{3}} [{1}, {2}]\n” deserves a few words. This lvalue requires the functions std::vformat and std::make_format_args. The number in the format string stands for the position in the arguments. The {3} is a placeholder for the longest time-zone name.

What’s Next?

I/O consists of the reading and writing of the chrono types. The various chrono types support the unformatted writing and the formatted one with the new formatting library. This library also has the function std::chrono::parse() that makes reading from a stream quite powerful.

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