Question

given a standard ISO 8601 timestamp, is there an easy way to get a time stamp from 20 minutes before and another time stamp of 20 minutes after the original timestamp?

For example given

 2010-01-25 18:02:00

I would want one or two functions to return,
2010-01-25 17:48:00 and 2010-01-25 18:22:00

The solution I gave up with is rather cumbersome, I use s.substr(14,2) to get the minutes and s.substr(12,2) to get the hours, convert the hour and minute strings to int and subtract or add the minute value with a condition if it goes above 60 or below 0. Is there a better/easier way to do this?

Answer 1

You could:

1. Convert the string into a struct tm using std::get_time.
2. Modify the struct tm (no need to consider the ranges).
3. Normalize the struct tm using mktime.
4. Print the struct tm using std::put_time

---

#include <ctime>
#include <string>
#include <sstream>
#include <iostream>
#include <iomanip>

std::string increase_minutes(const std::string& input, int delta) {
    struct tm tm;
    constexpr auto format = "%Y-%m-%d %T";

    std::istringstream iss (input);
    iss >> std::get_time(&tm, format);
    tm.tm_min += delta;
    mktime(&tm);

    std::ostringstream oss;
    oss << std::put_time(&tm, format);
    return oss.str();
}


int main() {
    auto s = "2012-12-31 23:59:00";
    std::cout << increase_minutes(s, 20) << std::endl;
    std::cout << increase_minutes(s, -20) << std::endl;
}

Note that std::get_time and std::put_time are absent in g++/libstdc++. You may need to parse the string using the C functions sscanf or strptime, and print the string using strftime. See also C++ date and time.

---

Edit: If we want to change by a generic std::chrono::duration, it is better we manipulate on a std::chrono::time_point directly. We could create a time_point from a struct tm with a two-step conversion: time_point ↔ time_t ↔ struct tm, as illustrated below:

#include <ctime>
#include <string>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <chrono>

template <typename R, typename P>
std::string increase_iso_time(const std::string& input, 
                              const std::chrono::duration<R, P>& duration) {
    struct tm tm;
    constexpr auto format = "%Y-%m-%d %T";

    std::istringstream iss (input);
    iss >> std::get_time(&tm, format);
    auto time = mktime(&tm);
    auto tp = std::chrono::system_clock::from_time_t(time);

    tp += duration;

    time = std::chrono::system_clock::to_time_t(tp);
    tm = *localtime(&time);
    std::ostringstream oss;
    oss << std::put_time(&tm, format);
    return oss.str();
}

int main() {
    auto s = "2012-12-31 23:59:00";
    std::cout << increase_iso_time(s, std::chrono::minutes(20)) << std::endl;
    std::cout << increase_iso_time(s, - std::chrono::hours(4)
                                      - std::chrono::minutes(20)) << std::endl;
}