string(TIMESTAMP): Fix %s placeholder for year 2038 on Windows

`difftime` returns a `double`, so commit 6727270b75 (CMake: Extend
TIMESTAMP sub-commands with new unix time format specifier, 2016-02-16,
v3.6.0-rc1~338^2) cast it to `long` to get an integer result.  However,
in the MSVC ABI, `long` is a 32-bit integer.  Use an explicit 64-bit
integer type instead.

Fixes: #26716

Source/cmTimestamp.cxx

@@ -16,6 +16,7 @@
 
 #include <cstdlib>
 #include <cstring>
+#include <ctime>
 #include <sstream>
 #include <utility>
 
@@ -275,7 +276,8 @@ std::string cmTimestamp::AddTimestampComponent(
         return std::string();
       }
 
-      return std::to_string(static_cast<long int>(difftime(timeT, unixEpoch)));
+      return std::to_string(
+        static_cast<int64_t>(std::difftime(timeT, unixEpoch)));
     }
     case 'f': // microseconds
     {

Tests/RunCMake/CMakeLists.txt

@@ -660,7 +660,10 @@ add_RunCMake_test(return)
 add_RunCMake_test(SarifOutput)
 add_RunCMake_test(separate_arguments)
 add_RunCMake_test(set_property)
-add_RunCMake_test(string)
+add_RunCMake_test(string
+  -DCMAKE_SYSTEM_NAME=${CMAKE_SYSTEM_NAME}
+  -DCMAKE_SYSTEM_PROCESSOR=${CMAKE_SYSTEM_PROCESSOR}
+  )
 add_RunCMake_test(test_include_dirs)
 add_RunCMake_test(BundleUtilities)
 if(APPLE)

Tests/RunCMake/string/RunCMakeTest.cmake

@@ -21,6 +21,14 @@ run_cmake(JoinNoArgs)
 run_cmake(JoinNoVar)
 
 run_cmake(Timestamp)
+if(NOT CMAKE_SYSTEM_NAME STREQUAL "AIX" # FIXME: Needs 64-bit build
+    AND NOT CMAKE_SYSTEM_NAME STREQUAL "SunOS" # FIXME: Needs 64-bit build
+    AND NOT (CMAKE_SYSTEM_NAME STREQUAL "Linux" AND
+    CMAKE_SYSTEM_PROCESSOR MATCHES "^(hppa|parisc64|sparc|sparc64)$" # FIXME: 32-bit time_t?
+             )
+    )
+  run_cmake(Timestamp2038)
+endif()
 run_cmake(TimestampEmpty)
 run_cmake(TimestampInvalid)
 run_cmake(TimestampInvalid2)

Tests/RunCMake/string/Timestamp2038-stderr.txt

@@ -0,0 +1 @@
+^RESULT=2038-01-20 05:00:00.000000 Wednesday=Wed January=Jan 38 day=020 wd=3 week=03 w_iso=03 %I=05 epoch=2147576400 TZ=GMT tz=\+0000$

Tests/RunCMake/string/Timestamp2038.cmake

@@ -0,0 +1,3 @@
+set(ENV{SOURCE_DATE_EPOCH} "2147576400")
+string(TIMESTAMP RESULT "%Y-%m-%d %H:%M:%S.%f %A=%a %B=%b %y day=%j wd=%w week=%U w_iso=%V %%I=%I epoch=%s TZ=%Z tz=%z" UTC)
+message("RESULT=${RESULT}")