Question

I read in two strings with a Year, the Julian Day (year day), hour, minute, and an observation.

I pull the relevant variables out using sscanf:

sscanf(tide_str1.c_str(), "%d %d %d %d %Lf", &y1, &j1, &h1, &m1, &obs1);
sscanf(tide_str2.c_str(), "%d %d %d %d %Lf", &y2, &j2, &h2, &m2, &obs2);

For this particular data set, the values are 2011 083 23 22 1.1

I then create and populate a tm structure, and run mktime, with cout calls on the day in between and it changes from 083 to 364.

int y1=2011, j1=83, h1=23, m1=22;
struct tm time_struct = {0, 0, 0, 0, 0, 0, 0, 0, 0}, *time_ptr = &time_struct;
time_t tv_min;
time_struct.tm_year = y1 - 1900;
time_struct.tm_yday = j1;
cout << time_struct.tm_yday << endl;
time_struct.tm_hour = h1;
time_struct.tm_min = m1;
time_struct.tm_isdst = -1;
cout << time_struct.tm_yday << endl;
tv_min = mktime(time_ptr);
cout << time_struct.tm_yday << endl;

Why is that? Is it because the tm_mday and and tm_mon are set to 0? I initially tried not initializing it all to zero, but then mktime returned -1. What should I be doing differently if I only know the year day and not the month and month day?

Answer 1

mktime() is doing what it’s supposed to do.

Quoting the C standard:

The mktime function converts the broken-down time, expressed as
local time, in the structure pointed to by timeptr into a calendar
time value with the same encoding as that of the values returned by
the time function. The original values of the tm_wday and
tm_yday components of the structure are ignored, and the original values of the other components are not restricted to the ranges
indicated above. On successful completion, the values of the
tm_wday and tm_yday components of the structure are set appropriately, and the other components are set to represent the
specified calendar time, but with their values forced to the ranges
indicated above; the final value of tm_mday is not set until tm_mon
and tm_year are determined.

mktime() can compute the values of tm_mday and tm_yday from other members; it isn’t designed to compute the values of other members from those fields.

What you can do, though, is initialize a struct tm with out-of-range values. For example, if you want tm_yday to be 200 (the 200th day of the year), you can initialize a struct tm representing the 200th day of January. mktime() will then normalize it to the correct date, yielding a time_t value that you can then feed to gmtime() or localtime().

Here’s a simple example:

#include <iostream>
#include <ctime>

int main()
{
    struct tm t = { 0 };
    t.tm_sec = t.tm_min = t.tm_hour = 0; // midnight
    t.tm_mon = 0;                        // January
    t.tm_year = 2012 - 1900;
    t.tm_isdst = -1;                     // unknown

    t.tm_mday = 200;                     // January 200th?

    time_t when = mktime(&t);
    const struct tm *norm = localtime(&when);   // Normalized time

    std::cout << "month=" << norm->tm_mon << ", day=" << norm->tm_mday << "\n";
    std::cout << "The 200th day of 2012 starts " << asctime(norm);
}

The output is:

month=6, day=18
The 200th day of 2012 starts Wed Jul 18 00:00:00 2012