Nuts and Volts - February 2013

Next, let's write Date Time Test2 and see how many seconds it has been since January 1, 1970:

int main()
{
  time_t myTime; // define a time_t variable
time(&myTime); // use as a pointer to get the
 // time in seconds
printf("It has been %ld seconds since Jan. 1
1970\n",myTime);

 return 0;
}

■ FIGURE 5. Seconds since epoch began.

Good grief, has it really been that long? It's 1,335,989,666 seconds already and it seems like only yesterday (which is me being sarcastic about how useful that number is to tell when now is). So, what is the date and time that this number represents? Well, let's use the asctime() function in Date Time3 to get the date and time in a format that makes sense:

int main()
{
  time_t myTime; // define a time_t variable
 
  time(&myTime); // use as a pointer to get
 // the time in seconds
 
  printf("The current time is
  %s.\n",asctime(localtime(&myTime)));
 
  return 0;
}

■ FIGURE 6. Date Time Test3.

We used the localtime function to convert the time since the beginning of the previously mentioned epoch into a calendar time, and then used the asctime() function to convert that into a string that we can then show on our console window using the printf function.

SMILEY’S WORKSHOP

Next, let's write Date Time Test4 to get the epoch time, and then convert that into the tm data structure so that we can manipulate each aspect of the date and time as separate variables:

int main()
{
  time_t myTime; // define a time_t variable
 
  time(&myTime); // use as a pointer to get
 // the time in seconds
 
  // load a pointer to a tm struct with the
  // local time
  struct tm* time_parts = localtime(&myTime);
 
  printf("Let's look at the data:\n");
  printf("tm_sec = %d\n",time_parts->tm_sec);
  printf("tm_min = %d\n",time_parts->tm_min);
  printf("tm_hour = %d\n",time_parts->tm_hour);
  printf("tm_mday = %d\n",time_parts->tm_mday);
  printf("tm_mon = %d\n",time_parts->tm_mon);
  printf("tm_year = %d\n",time_parts->tm_year);
  printf("tm_wday = %d\n",time_parts->tm_wday);
  printf("tm_yday = %d\n",time_parts->tm_yday);
  printf("tm_isdst = 
%d\n",time_parts->tm_isdst);

 return(0);
}

■ FIGURE 7. Show the tm structure.

Wait a second. The year is 112? Well, I neglected to mention that the year is calculated by adding the value in tm_year to the constant TM_YEAR_BASE that just happens to be 1900. So, 1900 + 112 = 2012 and we are right on time. You might notice that the month is 10, but in Date Time3 we saw that it is November. What gives?

Well, months are 0 based, so January is 0 and December is 11. We could take care of this in the software, but it is good to reinforce what we are doing by seeing the raw output.