diff --git a/BasiliskII/src/macos_util.cpp b/BasiliskII/src/macos_util.cpp
index 39071f8b..dea66451 100644
--- a/BasiliskII/src/macos_util.cpp
+++ b/BasiliskII/src/macos_util.cpp
@@ -137,7 +137,7 @@ uint32 TimeToMacTime(time_t t)
 	const int TM_EPOCH_YEAR = 1900;
 	const int MAC_EPOCH_YEAR = 1904;
 	// Clip year and day offsets to prevent dates earlier than 1-Jan-1904
-	local->tm_year = std::max(MAC_EPOCH_YEAR - TM_EPOCH_YEAR, local->tm_year - PrefsFindInt32("yearofs"));
+	local->tm_year = std::max(MAC_EPOCH_YEAR - TM_EPOCH_YEAR, local->tm_year + PrefsFindInt32("yearofs"));
 	int a4 = ((local->tm_year + TM_EPOCH_YEAR) >> 2) - !(local->tm_year & 3);
 	int b4 = (MAC_EPOCH_YEAR >> 2) - !(MAC_EPOCH_YEAR & 3);
 	int a100 = a4 / 25 - (a4 % 25 < 0);
@@ -146,7 +146,7 @@ uint32 TimeToMacTime(time_t t)
 	int b400 = b100 >> 2;
 	int intervening_leap_days = (a4 - b4) - (a100 - b100) + (a400 - b400);
 	uint32 days = local->tm_yday + 365 * (local->tm_year - 4) + intervening_leap_days;
-	int32 dayofs = PrefsFindInt32("dayofs");
+	int32 dayofs = -PrefsFindInt32("dayofs");
 	if(dayofs > 0 && dayofs > days)
 		dayofs = days;
 	return local->tm_sec + 60 * (local->tm_min + 60 * (local->tm_hour + 24 * (days - dayofs)));
diff --git a/SheepShaver/src/macos_util.cpp b/SheepShaver/src/macos_util.cpp
index a00af4ba..87eabb3d 100644
--- a/SheepShaver/src/macos_util.cpp
+++ b/SheepShaver/src/macos_util.cpp
@@ -336,7 +336,7 @@ uint32 TimeToMacTime(time_t t)
 	const int TM_EPOCH_YEAR = 1900;
 	const int MAC_EPOCH_YEAR = 1904;
 	// Clip year and day offsets to prevent dates earlier than 1-Jan-1904
-	local->tm_year = std::max(MAC_EPOCH_YEAR - TM_EPOCH_YEAR, local->tm_year - PrefsFindInt32("yearofs"));
+	local->tm_year = std::max(MAC_EPOCH_YEAR - TM_EPOCH_YEAR, local->tm_year + PrefsFindInt32("yearofs"));
 	int a4 = ((local->tm_year + TM_EPOCH_YEAR) >> 2) - !(local->tm_year & 3);
 	int b4 = (MAC_EPOCH_YEAR >> 2) - !(MAC_EPOCH_YEAR & 3);
 	int a100 = a4 / 25 - (a4 % 25 < 0);
@@ -345,7 +345,7 @@ uint32 TimeToMacTime(time_t t)
 	int b400 = b100 >> 2;
 	int intervening_leap_days = (a4 - b4) - (a100 - b100) + (a400 - b400);
 	uint32 days = local->tm_yday + 365 * (local->tm_year - 4) + intervening_leap_days;
-	int32 dayofs = PrefsFindInt32("dayofs");
+	int32 dayofs = -PrefsFindInt32("dayofs");
 	if(dayofs > 0 && dayofs > days)
 		dayofs = days;
 	return local->tm_sec + 60 * (local->tm_min + 60 * (local->tm_hour + 24 * (days - dayofs)));