gh-100388: Change undefined __DATE__ to the Unix epoch (#100389)

Misc/NEWS.d/next/Core_and_Builtins/2022-12-21-14-28-01.gh-issue-100388.vne8ky.rst

@@ -0,0 +1,2 @@
+Fix the ``platform._sys_version()`` method when ``__DATE__`` is undefined at
+buildtime by changing default buildtime datetime string to the UNIX epoch.

Modules/getbuildinfo.c

@@ -13,7 +13,7 @@
 #ifdef __DATE__
 #define DATE __DATE__
 #else
-#define DATE "xx/xx/xx"
+#define DATE "Jan 01 1970"
 #endif
 #endif
 
@@ -21,7 +21,7 @@
 #ifdef __TIME__
 #define TIME __TIME__
 #else
-#define TIME "xx:xx:xx"
+#define TIME "00:00:00"
 #endif
 #endif