Microsoft Year 2000 Readiness Disclosure & Resource Center

ANSI Time and Visual C++ libraries

1. struct tm
2. time_t

ANSI types: struct tm and time_t

The ANSI C standard has established these types, their meaning and to some extent, the underlying type of each. In Visual C++, the type of time_t is long and it represents seconds since January 1, 1970 when used with the ANSI C functions that utilize variables of this type. The type struct tm is used to track the components of a date. The struct tm uses internal variables to store years as the number of years since 1900, the month of the year, day of the week, day of the month, day of the year, hour of the day, minute of the hour, and seconds of the minute are accounted for in this structure as well as if Daylight Savings Time is in effect. These members are of type int. These types are what ANSI functions use to manipulate, input, and output times. The Visual C++ documentation has more information on these types.

Avoiding issues with ANSI time functions

An application can appear to have a year 2000 issue because of how ANSI functions are permitted to behave. You can create year 2000 compliant applications and avoid confusing output from your programs if you follow some simple guidelines when using ANSI functions, primarily the strftime and wcsftime functions.

1. Present 4-digit years when displaying dates to users of your application. When using the strftime and wcsftime functions, use %Y to display a 4-digit year instead %y (lowercase) which displays a 2-digit year. For example:

   #include <time.h>
   #include <stdio.h>
   const int BUFSIZE = 256;
   char buf[BUFSIZE];
   int main()
   {
   struct tm now;
   time_t tmp = time(0); // get current time
   now = *localtime(&tmp); // get components
   
   // 2-digit vs. 4-digit year
   char fmt[] = " %m/%d/%y \n %m/%d/%Y ";
   strftime( buf, BUFSIZ, fmt, &now );
   printf(buf);
   
   return 0;
   }
   

2. Require users of your application to provide fully qualified dates (using 4-digit years) when accepting date input into your application.
3. Avoid using the %x or %c format, or if used, use # to get the long date version when using strftime and wcsftime. The %x and %c formatting codes default to printing a 2-digit year. Change the initialization of fmt in the above program to " %x \n %#x " to see the difference.

   Keep your times in a time_t variable or a struct tm and manipulate them there. ANSI C mandates some other time functions to aid in this task. Here is a list and brief description of these functions. See your Visual C++ documentation for more details.

   • time û gets the current calendar time in a time_t
   • difftime û computes the difference between two time_t's
   • gmtime û converts a time_t to a struct tm, in terms of Coordinated Universal Time (UTC)
   • localtime û converts a time_t to a struct tm, in terms of local time
   • asctime, _wasctime û returns a string corresponding to the passed struct tm*. The string is of the form "DDD MMM dd HH:MM:SS YYYY\n\0" and is always 26 characters.
   • ctime, _wctimeû given a time_t, returns the local time in the form of a string. Exactly the same as asctime( localtime( time_t_val ))
   • mktime û returns a locally adjusted time_t given a struct tm*, which is updated so values fall into appropriate ranges. For example

     #include <time.h>
     #include <stdio.h>
     const int BUFSIZE = 1024;
     char buf[BUFSIZE];
     int main()
     {
     struct tm now;
     time_t tmp = time(0); // current time
     now = *localtime(&tmp);
     size_t len = strftime(buf, BUFSIZE, "%#c\n", &now);
     
     now.tm_hour += 48; //jump ahead two days
     
     // should get a bogus hour value
     len += strftime(buf+len, BUFSIZE-len, "%#c\n", &now);
     
     mktime(&now); // mktime will correct
     strftime(buf+len, BUFSIZE-len, "%#c\n", &now);
     
     printf(buf);
     return 0;
     }
     

4. Although not an ANSI function, the _strdate and _wstrdate functions generates a 9-character string to a buffer that it receives a pointer to as a parameter. The output has the form "MM/DD/YY". Use the strftime or wcsftime function instead.

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