diff --git a/AerofoilPortable/GpSystemServices_POSIX.cpp b/AerofoilPortable/GpSystemServices_POSIX.cpp
index 1566a4f..15bcb55 100644
--- a/AerofoilPortable/GpSystemServices_POSIX.cpp
+++ b/AerofoilPortable/GpSystemServices_POSIX.cpp
@@ -17,7 +17,7 @@ GpSystemServices_POSIX::GpSystemServices_POSIX()
 int64_t GpSystemServices_POSIX::GetTime() const
 {
 	time_t t = time(nullptr);
-	return static_cast<int64_t>(t) - 2082844800;
+	return static_cast<int64_t>(t) + 2082844800;
 }
 
 void GpSystemServices_POSIX::GetLocalDateTime(unsigned int &year, unsigned int &month, unsigned int &day, unsigned int &hour, unsigned int &minute, unsigned int &second) const
diff --git a/MakeTimestamp/MakeTimestamp.cpp b/MakeTimestamp/MakeTimestamp.cpp
index 58694d8..1e82169 100644
--- a/MakeTimestamp/MakeTimestamp.cpp
+++ b/MakeTimestamp/MakeTimestamp.cpp
@@ -71,7 +71,7 @@ int main(int argc, const char **argv)
 	}
 
 	PortabilityLayer::CombinedTimestamp ts;
-	ts.SetMacEpochTime(currentTimeUnix + ts.kMacEpochToUTC);
+	ts.SetMacEpochTime(static_cast<int64_t>(currentTimeUnix) - ts.kMacEpochToUTC);
 
 	ts.SetLocalYear(currentTimeStruct->tm_year + 1900);
 	ts.m_localMonth = currentTimeStruct->tm_mon + 1;
diff --git a/PortabilityLayer/CombinedTimestamp.h b/PortabilityLayer/CombinedTimestamp.h
index 85ea457..7766e6f 100644
--- a/PortabilityLayer/CombinedTimestamp.h
+++ b/PortabilityLayer/CombinedTimestamp.h
@@ -18,7 +18,7 @@ namespace PortabilityLayer
 
 		uint8_t m_padding[3];
 
-		static const int32_t kMacEpochToUTC = 2082844800;
+		static const int32_t kMacEpochToUTC = -2082844800;
 
 		int64_t GetMacEpochTime() const;
 		void SetMacEpochTime(int64_t timestamp);