EPOCHREALTIME does not behave correctly before 1970

[Emanuele Torre]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -march=x86-64 -mtune=generic -O2 -pipe -fno-plt
-DDEFAULT_PATH_VALUE='/usr/local/
uname output: Linux t420 5.10.54-1-lts #1 SMP Wed, 28 Jul 2021
15:05:20 +0000 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.1
Patch Level: 8
Release Status: release

Description:

>  static SHELL_VAR *
>  get_epochrealtime (var)
>       SHELL_VAR *var;
>  {
>    char buf[32];
>    char *p;
>    struct timeval tv;
>
>    gettimeofday (&tv, NULL);
>    snprintf (buf, sizeof (buf), "%u%c%06u", (unsigned)tv.tv_sec,
>     locale_decpoint (),
>     (unsigned)tv.tv_usec);
>
>    p = savestring (buf);
>    FREE (value_cell (var));
>    var_setvalue (var, p);
>    return (var);
>  }

get_epochrealtime() casts tv.tv_sec (a time_t a.k.a. int) to unsigned
int causing EPOCHREALTIME to not behave correctly before the Unix Epoch.

> APPLICATION USAGE
>  Applications should use the clock_gettime() function instead of the
>  obsolescent gettimeofday() function.
> [...]
> FUTURE DIRECTIONS
>  The gettimeofday() function may be removed in a future version.
> [...]
> Issue 7
>  The gettimeofday() function is marked obsolescent.

I have also read that gettimeofday() is considered obsolete and is
deprecated by POSIX.

See: 
https://pubs.opengroup.org/onlinepubs/9699919799/functions/gettimeofday.html

Repeat-By:

  $ faketime 1969-01-01 bash -c 'declare -p EPOCHSECONDS EPOCHREALTIME'
  declare -- EPOCHSECONDS="-31539600"
  declare -- EPOCHREALTIME="4263427696.758887"