Fix for #432.

source/parser/parser_expressions.cpp

@@ -1115,7 +1115,41 @@ void Parser::Parse_Num_Factor (EXPRESS& Express,int *Terms)
                     {
                         static boost::posix_time::ptime y2k(boost::gregorian::date(2000,1,1));
                         boost::posix_time::ptime now(boost::posix_time::microsec_clock::universal_time());
-                        Val = (now-y2k).total_microseconds() * (1.0e-6) / (24*60*60);
+
+                        // Due to a bug in `boost::posix_time::microsec_clock::universal_time()`,
+                        // the value returned may actually be local time rather than UTC. We try to fix this
+                        // by comparing with `boost::posix_time::second_clock::universal_time()`, which is
+                        // less precise but more reliable in terms of time zone behavior.
+                        // (NB: While we could theoretically compute the offset once, and then re-use it every time
+                        // `new` is invoked, this would cause issues when rendering a scene during transition to or
+                        // from daylight savings time, or other events affecting the time zone. The current approach
+                        // is immune to such issues.)
+                        boost::posix_time::ptime lowPrecisionNow(boost::posix_time::second_clock::universal_time());
+                        // The difference between the two timers, rounded to quarters of an hour,
+                        // should correspond to the time zone offset (in seconds in this case).
+                        const auto tzPrecisionInSeconds = 15 * 60;
+                        int_least32_t tzOffset = std::lround(float((now - lowPrecisionNow).total_seconds()) / tzPrecisionInSeconds) * tzPrecisionInSeconds;
+                        // Unless someone paused the code in between the above statements, the resulting difference
+                        // should be our time zone offset in seconds (unless we're running on a system that's not
+                        // subject to the bug, in which case the resulting difference should be zero).
+                        if (tzOffset != 0)
+                        {
+                            if (sceneData->EffectiveLanguageVersion() < 380)
+                            {
+                                WarningOnce(kWarningGeneral, HERE,
+                                    "In POV-Ray v3.7, on some platforms 'now' erroneously evaluated to days since "
+                                    "2000-01-01 00:00 local time instead of UTC. For backward compatibility, this "
+                                    "bug is reproduced in legacy scenes, but the scene may produce different "
+                                    "results on other platforms.");
+                            }
+                            else
+                            {
+                                now -= boost::posix_time::time_duration(0, 0, tzOffset);
+                            }
+                        }
+
+                        const auto daysPerMicrosecond = 1.0e-6 / (24 * 60 * 60);
+                        Val = (now - y2k).total_microseconds() * daysPerMicrosecond;
                     }
                     break;
             }