What is the best way to convert UnixTime to a date?
Is there a function for it or an algorithm?
Unix time is seconds since epoch (1970-01-01). Depending on what you mean, you can convert it to a struct tm with localtime or convert it to a string with strftime.
time_t t = time(NULL);
struct tm *tm = localtime(&t);
char date[20];
strftime(date, sizeof(date), "%Y-%m-%d", tm);
As the manual to localtime states
The return value points to a statically allocated struct which might be overwritten by subsequent calls to any of the date and time functions.
This is what some refer to as data races. This happens when two or more threads call localtime simultaneously.
To protect from this, some suggest using localtime_s, which is a Microsoft only function. On POSIX systems, you should use localtime_r instead
The localtime_r() function does the same, but stores the data in a user-supplied struct.
Usage would look like
time_t t = time(NULL);
struct tm res;
localtime_r(&t, &res);