Your value 1643873040 appears to be a Unix time value. It's unusual to have it as a string, because it's obviously an integer. So the first thing to do is to convert it to an actual integer. The traditional type in C for this sort of thing is time_t. (It used to be 32 bits, but these days it's usually 64 bits.)
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
const char *sDateTime = "1643873040";
time_t t = strtoll(sDateTime, NULL, 10);
Next, to convert that number t into a human-readable format, the first step is to use the function localtime, which returns a pointer to a "broken down" time structure, struct tm:
struct tm *tmp = localtime(&t);
Next, you can use the strftime function to create a string from the struct tm:
char strbuf[30];
strftime(strbuf, sizeof(strbuf), "%Y-%m-%d %H:%M:%S", tmp);
printf("%s\n", strbuf);
This prints:
2022-02-03 02:24:00
Or, you can print the fields from the struct tm directly:
printf("%d-%02d-%02d %02d:%02d:%02d\n",
tmp->tm_year+1900, tmp->tm_mon+1, tmp->tm_mday,
tmp->tm_hour, tmp->tm_min, tmp->tm_sec);
Here, though, you have to be careful, because the tm_mon field is 0-based, and tmp_year is offset by 1900.
These are the ways I always do it, although it must be admitted that the localtime function is problematic by modern standards, in that it returns a pointer to static data and is not therefore thread-safe. There's a thread-safe or "reentrant" version you can use instead:
struct tm tmbuf;
tmp = localtime_r(&t, &tmbuf);
(Or, as you've learned, there's a different alternative version, localtime_s, although I'm not aware of any advantage it might have over localtime_r.)
Finally, for completeness I might mention that there's also a simpler way, that goes straight from the time_t value to a string, without any localtime calls or a struct tm at all:
printf("%s", ctime(&t));
This prints
Thu Feb 3 02:24:00 2022
but it's quite a bit less flexible, because there's no way to change it. ctime's output format is fixed.
A time_t value like 1643873040 usually represents UTC, also known as GMT. The localtime function takes care of converting it to your local time zone (including DST corrections, if necessary). If you want UTC time instead, without applying time zone and DST corrections, you can call gmtime instead of localtime.
This answer has been for C. The localtime, gmtime, and strftime functions will work fine in C++ also, although C++ has its own, perhaps preferred ways of doing it.
