How Do I Get Yesterday's Date In C?

Question

I am wanting to get yesterday's date into a char in the format: YYYYMMDD (with no slashes dots etc.).

I am using this code to get today's date:

time_t now;

struct tm  *ts;  
char yearchar[80]; 

now = time(NULL);  
ts = localtime(&now);

strftime(yearchar, sizeof(yearchar), "%Y%m%d", ts);

How would I adapt this code so that it is generating yesterday's date instead of today's?

Many Thanks.

Answer 1

The mktime() function will normalise the struct tm that you pass it (ie. it will convert out-of-range dates like 2020/2/0 into the in-range equivalent 2020/1/31) - so all you need to do is this:

time_t now;
struct tm  *ts;  
char yearchar[80]; 

now = time(NULL);
ts = localtime(&now);
ts->tm_mday--;
mktime(ts); /* Normalise ts */
strftime(yearchar, sizeof(yearchar), "%Y%m%d", ts);

Answer 2

how about adding

now = now - (60 * 60 * 24)

Might fail in some VERY rare corner cases (e.g. during leapseconds) but should do what you want 99.999999% of the time.

Answer 3

Simply subtracting one day's worth of seconds from time(NULL); should do. Change this line:

now = time(NULL);

to this:

now = time(NULL) - (24 * 60 * 60);