Allow document revision time to be set from the environment

This can be used to get reproducible HTML output, making it easier to
verify that changes to the code do not introduce unwanted changes in the
HTML.

Author: jwakely

src/report_generator.cpp

@@ -11,6 +11,7 @@
 #include <algorithm>
 #include <cassert>
 #include <chrono>
+#include <cstdlib>
 #include <format>
 #include <fstream>
 #include <memory>
@@ -25,7 +26,15 @@ namespace
 // Generic utilities that are useful and do not rely on context or types from our domain (issue-list processing)
 // =============================================================================================================
 
-auto const timestamp{std::chrono::floor<std::chrono::seconds>(std::chrono::system_clock::now())};
+auto const timestamp = [] {
+  // Allow the timestamp to be set from a unix timestamp in the environment,
+  // to support reproducible HTML output. For example:
+  // LWG_REVISION_TIME=$(date +%s -d "2025-10-07 at 09:38:29 UTC") make lists
+  if (const char* revtime = std::getenv("LWG_REVISION_TIME"))
+    return std::chrono::sys_seconds(std::chrono::seconds(std::stol(revtime)));
+  auto now = std::chrono::system_clock::now();
+  return std::chrono::floor<std::chrono::seconds>(now);
+}();
 
 // global data - would like to do something about that.
 std::string const build_date{std::format("{:%F}", timestamp)};