mktime issues

[strider1974]

I am doing a project and I need to be able to display a simple calendar for the month and year supplied by user input.
To do this I have written the following to determine the first day of the month selected.

int getFirstDayOFMonth(unsigned year, unsigned month) {

   struct tm time_struct;
   
   time_struct.tm_year = year - 1900;
   time_struct.tm_mon = month - 1;
   time_struct.tm_mday = 1;
   time_struct.tm_sec = 0; 
   time_struct.tm_min = 0;
   time_struct.tm_hour = 0;
   time_struct.tm_isdst = 0;

   if (mktime(&time_struct) == -1)
      return -1;
   else 
      return time_struct.tm_wday;
}

My problem arises when I enter a year larger than 2038 and smaller than 1902. Reading through this forum I found out about the Year 2038 problem so I understand the upper limit but I cannot understand what the issue is with 1900 and 1901.

Is there any way to get around the 2038 problem for the case above?
Why is mktime() failing on the years 1900 and 1901? Is there a way around this?
Is there a way you can you go below 1900?

[matsp]

The "time" system in Unix is defined in such a way that it's not easy to do anything about those matters - time is a 32-bit integer, and 2^32 (4 billion) seconds is the entire range - it goes backwards and forwards 2^31 (+1 backwards) seconds, which means you can go 2147 million (and a bit) seconds either side of 1st Jan 1970.-

Sun's man-page here says:
[quote="Sun"]
The tm_year member must be for year 1901 or later. Calendar times before 20:45:52 UTC, December 13, 1901 or after 03:14:07 UTC, January 19, 2038 cannot be represented. Portable applications should not try to create dates before 00:00:00 UTC, January 1, 1970 or after 00:00:00 UTC, January 1, 2038.
[/quite]

If you need a greater range than that, you will need a different form to store your time - you could simply extend it to 64-bit, and then you would be able to go about 130 billion years or so in either direction.

--
Mats

[strider1974]

cheers for the quick response matsp

How can I extend to 64-bit?

[matsp]

If your system has "mktime64", then you can use that (and the related time64, difftime64 etc). If there is no mktime64 on your system, then you have to roll your own (or change to a system that supports 64-bit time?).

--
Mats