Fix for POV-Ray#432.

To work around Boost's msec-precision UTC timer erroneously reporting local time instead of UTC on some systems, we're now also probing the sec-precision UTC timer, which should be more reliable in terms of time zone: The rounded difference between the two should be able to give us the time zone offset (if any) in the msec-precision timer, which we then compensate for accordingly.

source/parser/parser_expressions.cpp

@@ -1115,7 +1115,22 @@ 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.
+                        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.
+                        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;
             }