C++ 将日、月、年转换为毫秒

Question

I need to convert M:D:Y:H:M:S into milliseconds.

This is the function for Arduino but can't get the same in c++ for Raspberry Pi 3.

time_t tmConvert_t(int YYYY, byte MM, byte DD, byte hh, byte mm, byte ss)
{
  tmElements_t tmSet;
  tmSet.Year = YYYY - 1970;
  tmSet.Month = MM;
  tmSet.Day = DD;
  tmSet.Hour = hh;
  tmSet.Minute = mm;
  tmSet.Second = ss;
  return makeTime(tmSet); 
}

Any time I need a date converted to milliseconds I run this line.

unsigned long midnight = tmConvert_t(currentYear, currentMonth, currentDay, 0,0,0);  // get current start of day time stamp

Thanks

Answer 1

You could just do what you are doing right now if you know time_t to be an arithmetic type holding the number of seconds passed since the epoch. A portable way could be to instead let std::chrono do the calculation for you.

#include <chrono>
#include <ctime>
#include <iostream>

std::chrono::milliseconds::rep
tmConvert_t(int YYYY, int MM, int DD, int hh, int mm, int ss) {
    std::tm t{};             // initialize

    t.tm_year = YYYY - 1900;
    t.tm_mon = MM - 1;
    t.tm_mday = DD;
    t.tm_hour = hh;
    t.tm_min = mm;
    t.tm_sec = ss;
    time_t orig = std::mktime(&t);
    
    // ask for the duration since the epoch
    auto dur = std::chrono::system_clock::from_time_t(orig).time_since_epoch();

    // cast to whole milliseconds
    auto dur_in_ms = std::chrono::duration_cast<std::chrono::milliseconds>(dur);

    // return the tick count in milliseconds
    return dur_in_ms.count();
}

int main() {
    std::cout << tmConvert_t(1970,01,01,0,0,0) << '\n'; // possible epoch
    std::cout << tmConvert_t(2021,02,15,0,0,0) << '\n'; // Today
}

Demo

Answer 2

This is the same as above but works with only c++.

unsigned long long tmConvert_t(short YY, short MM, short DD, short hh, short mm, short ss)
{
    std::tm example;
    example.tm_sec = ss;
    example.tm_min = mm;
    example.tm_hour = hh;
    example.tm_year = YY - 1900;
    example.tm_mon = MM;
    example.tm_mday = DD;
    unsigned long long milli = std::mktime(&example);
    milli = milli * 1000;
    return milli; 
}

Then call this to get milliseconds since epoch.

unsigned long long midnightMillis = tmConvert_t(2021,01,13,0,0,00);

Thanks for all the help.