Convert char array representing a date to seconds since 1970

Question

There are many links to achieve this but I can't actually find a code example from the accepted answers (that doesn't use boost), what would be the most efficient way to have something such as this:

char date[] = "2021-01-01";
long double seconds = SecondsSince1970(date);

and an output of something like 10000000000 or however many seconds that is?

Edit: (achieved in C++ by)

std::tm t = {};
std::istringstream ss("2010-11-04T23:23:01Z");
if (ss >> std::get_time(&t, "%Y-%m-%dT%H:%M:%S")){
    std::cout << std::put_time(&t, "%c") << "\n"  << std::mktime(&t) << "\n";
}

I guess an extension to this question is how can I replicate this C++ code but without the use of istringstream so it doesn't have to allocate memory, rather it just goes directly from the raw char [] array to seconds?

Uh, I need it to be exact with the current time, so when I convert back its the same. For example, in python, i just tried `time.time()` and got `1616828423.9408524` so something along those lines — , Mar 27 '21 at 07:00
Does this answer your question? [strtotime function in C](https://stackoverflow.com/questions/30486776/strtotime-function-in-c) — r3mainer, Mar 27 '21 at 07:08
@AnttiHaapala I've seen that, im on windows, I can't get that working, also any of those answers don't have any examples of how to use them other than just saying use that function. I want to be able to do it with a `char []` so theres no new memory allocation to an `istringstream`, is that possible? — , Mar 27 '21 at 07:52
then you need to **edit** your question and add that information to the question. — Antti Haapala -- Слава Україні, Mar 27 '21 at 08:00
if you do not need to validate the format you can use `sscanf` and `mktime`. — Antti Haapala -- Слава Україні, Mar 27 '21 at 08:03
Like in your C++ example. You know how to use `sscanf`? If not google "parse date using sscanf` :D For example [this](https://stackoverflow.com/questions/12071342/reading-a-date-with-sscanf) — Antti Haapala -- Слава Україні, Mar 27 '21 at 08:05
the date is not assigned to any variables or integers, its just an array of characters, i just need to convert that to seconds since 1970 as python does — , Mar 27 '21 at 08:07

David Ranieri · Accepted Answer · 2021-03-27T08:21:34.413

2

You can do it in 3 simple steps:

sscanf the string (Bonus - Check valid date)
Get the days since 1970
Multiply by 24 * 60 * 60

#include <stdio.h>

int leap_years(int month, int year)
{
    int years = year;

    if (month <= 2)
    {
        years--;
    }
    return (years / 4) - (years / 100) + (years / 400);
}

int days_diff(int day1, int month1, int year1,
              int day2, int month2, int year2)
{
    static const int days[] = {0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334};
    long int days1 = (year1 * 365) + day1;
    long int days2 = (year2 * 365) + day2;

    days1 += days[month1 - 1] + leap_years(month1, year1);
    days2 += days[month2 - 1] + leap_years(month2, year2);
    return (int)(days2 - days1);
}

int year_isleap(int year)
{
    return (((year % 4) == 0) && ((year % 100) != 0)) || ((year % 400) == 0);
}

int month_days(int month, int year)
{
    static const int days[2][12] =
    {
        {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
        {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
    };
    int leap = year_isleap(year);

    return days[leap][month - 1];
}

int date_isvalid(int day, int month, int year)
{
    if ((year < 0) || (year > 9999))
    {
        return 0;
    }
    if ((month < 1) || (month > 12))
    {
        return 0;
    }
    if ((day < 1) || (day > month_days(month, year)))
    {
        return 0;
    }
    return 1;
}

int main(void)
{
    int d, m, y;

    if ((sscanf("2021-01-01", "%d-%d-%d", &y, &m, &d) == 3) &&
        (date_isvalid(d, m, y)))
    {
        long double seconds = days_diff(1, 1, 1970, d, m, y) * 24 * 60 *60;

        printf("%.0Lf\n", seconds);
    }
    else
    {
        puts("Wrong date");
    }
    return(0);
}

Output:

1609459200

edited Mar 27 '21 at 08:21

answered Mar 27 '21 at 07:17

David Ranieri

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Sorry, I'll edit the question. I think your answer is very useful but I should have specified a bit clearer. Please leave it though, I'm sure ill need it in the future – Mar 27 '21 at 07:20
@AndySukowski-Bang it would be a bit difficult to get the number of seconds since epoch from `"2021-01-01"` with `time(NULL)`? – David C. Rankin Mar 27 '21 at 07:28
Wrong David `:)` (too many of them around tonight) – David C. Rankin Mar 27 '21 at 07:32
What about, with my edit, the C++ way but without `istringstream`? Is that possible, I want to directly achieve it from my `char []` so theres no allocations after that, just straight from `char []` to seconds? – Mar 27 '21 at 07:37
Can you update it so it accounts for leap years such as python `time.time()` to return the same result? Sorry i know this might be a bit complicated I just need that time as specific as that since im going over data from history where it may or may not be in 29th feb and what not. I know you put leap year, but that doesn't return the same time as `time.time()` in python/ – Mar 27 '21 at 07:50
@prtwrt I don't get you, leap years are also calculated and I'm not familiar with Python, what is the exact expected result? – David Ranieri Mar 27 '21 at 07:54
um, im not quite sure how to implement exact time into your answer, but say i input the exact time it is now, I need it to return exactly `1616831759.302955` for 6:56PM (around 10-20s), 27 march 2021 AEST – Mar 27 '21 at 07:56
@prtwrt Why is it a floating point number? – Andy Sukowski-Bang Mar 27 '21 at 07:59
i guess milliseconds, yeah sorry disregard that decimal – Mar 27 '21 at 07:59

Convert char array representing a date to seconds since 1970

1 Answers1