C: How to generate a date structure

Question

good afternoon.

I’m having a hard time with the C-date manipulation. I’m using the time. h library to mess with dates.

I need to create a struct tm which contains the date of the day (example) 29/01/1950 and then save its value time_t in a variable.
The problem is that I create the structure, assign the values of the day/month/year to their respective variables (tm_mday, tm_mon and tm_year), but when I use the function mktime(), he returns me -1.

Excerpt from the code:

data->tm_mday = dia;
data->tm_mon = mes - 1;
data->tm_year = ano - 1900;
segs = mktime(data);

I was not having problems with this solution for recent dates.
What might be going on?

EDIT 1: Running a loop to see how far mktime() works, I realized that it works until the value of tm_year equals 70. Below this it returns me -1...

EDIT 2: Doing a few more tests, I noticed that the value that mktime() returns is the number of seconds since 31/12/1969 22:00. Any date before that I can’t create with that structure.

Answer 1

mktime() uses the "Unix era", which is a way of representing dates by means of seconds. Where a date (along with hour, minutes and seconds) is represented by the number of seconds between 1 January 1970 at 00:00:00 (UTC) and the desired date. Therefore any date prior to this will return an error.