fix time format string for use with MinGW; use std::chrono::system_clock

Log.cpp

@@ -161,11 +161,16 @@ void Entry::addMetadataHeader() {
 	static const char* LevelTag[] = {"",         "[FATAL] ", "[ERROR] ", "[WARN ] ",
 	                                 "[NOTE ] ", "[INFO ] ", "[DEBUG] ", "[TRACE] "};
 
-	// get current date/time
-	std::time_t now = std::time(NULL);
-
 	// datetime
-	ss << "[" << std::put_time(std::localtime(&now), "%F %T") << "]";
+	using namespace std::chrono;
+	auto now = system_clock::to_time_t(system_clock::now());
+	auto tm = *std::localtime(&now);
+
+	// MinGW doesn't support the ISO8601 formatting characters like "%F" and "%T"
+	// ref: https://sourceforge.net/p/mingw-w64/bugs/793/
+	// Therefore, use a more verbose time string
+	ss << "[" << std::put_time(&tm, "%Y-%m-%d %H:%M:%S") << "]";
+
 	// log level
 	ss << LevelTag[lvl];
 }