Define Git and date constants in executables only (#4681)

CMakeLists.txt

@@ -185,6 +185,16 @@ if (BUILD_WITH_CRASHPAD)
     add_subdirectory("${CMAKE_SOURCE_DIR}/lib/crashpad" EXCLUDE_FROM_ALL)
 endif()
 
+# Used to provide a date of build in the About page (for nightly builds). Getting the actual time of
+# compilation in CMake is a more involved, as documented in https://stackoverflow.com/q/24292898.
+# For CI runs, however, the date of build file generation should be consistent with the date of
+# compilation so this approximation is "good enough" for our purpose.
+if (DEFINED ENV{CHATTERINO_SKIP_DATE_GEN})
+    set(cmake_gen_date "1970-01-01")
+else ()
+    string(TIMESTAMP cmake_gen_date "%Y-%m-%d")
+endif ()
+
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)