diff --git a/src/common/TimeZoneUtil.cpp b/src/common/TimeZoneUtil.cpp
index d806f0b46d..b3ba461bdb 100644
--- a/src/common/TimeZoneUtil.cpp
+++ b/src/common/TimeZoneUtil.cpp
@@ -292,6 +292,20 @@ ISC_TIMESTAMP TimeZoneUtil::timeStampTzToTimeStamp(const ISC_TIMESTAMP_TZ& timeS
 	return TimeStamp::encode_timestamp(&times, fractions);
 }
 
+// Converts a time from local to UTC.
+void TimeZoneUtil::localTimeToUtc(ISC_TIME& time, Callbacks* cb)
+{
+	//// TODO:
+	ISC_TIMESTAMP_TZ tempTimeStampTz;
+	tempTimeStampTz.timestamp_date = TimeStamp::getCurrentTimeStamp().value().timestamp_date;
+	tempTimeStampTz.timestamp_time = time;
+	tempTimeStampTz.timestamp_zone = cb->getSessionTimeZone();
+
+	localTimeStampToUtc(tempTimeStampTz);
+
+	time = tempTimeStampTz.timestamp_time;
+}
+
 // Converts a time from local to UTC.
 void TimeZoneUtil::localTimeToUtc(ISC_TIME_TZ& timeTz)
 {
@@ -306,6 +320,20 @@ void TimeZoneUtil::localTimeToUtc(ISC_TIME_TZ& timeTz)
 	timeTz.time_time = tempTimeStampTz.timestamp_time;
 }
 
+// Converts a timestamp from its local datetime fields to UTC.
+void TimeZoneUtil::localTimeStampToUtc(ISC_TIMESTAMP& timeStamp, Callbacks* cb)
+{
+	ISC_TIMESTAMP_TZ tempTimeStampTz;
+	tempTimeStampTz.timestamp_date = timeStamp.timestamp_date;
+	tempTimeStampTz.timestamp_time = timeStamp.timestamp_time;
+	tempTimeStampTz.timestamp_zone = cb->getSessionTimeZone();
+
+	localTimeStampToUtc(tempTimeStampTz);
+
+	timeStamp.timestamp_date = tempTimeStampTz.timestamp_date;
+	timeStamp.timestamp_time = tempTimeStampTz.timestamp_time;
+}
+
 // Converts a timestamp from its local datetime fields to UTC.
 void TimeZoneUtil::localTimeStampToUtc(ISC_TIMESTAMP_TZ& timeStampTz)
 {
diff --git a/src/common/TimeZoneUtil.h b/src/common/TimeZoneUtil.h
index 4ac302a82b..43b040ac73 100644
--- a/src/common/TimeZoneUtil.h
+++ b/src/common/TimeZoneUtil.h
@@ -28,6 +28,7 @@
 #define COMMON_TIME_ZONE_UTIL_H
 
 #include "../common/classes/fb_string.h"
+#include "../common/cvt.h"
 
 namespace Firebird {
 
@@ -53,7 +54,10 @@ public:
 	static ISC_TIME timeTzToTime(const ISC_TIME_TZ& timeTz, USHORT toTimeZone);
 	static ISC_TIMESTAMP timeStampTzToTimeStamp(const ISC_TIMESTAMP_TZ& timeStampTz, USHORT toTimeZone);
 
+	static void localTimeToUtc(ISC_TIME& time, Callbacks* cb);
 	static void localTimeToUtc(ISC_TIME_TZ& timeTz);
+
+	static void localTimeStampToUtc(ISC_TIMESTAMP& timeStamp, Callbacks* cb);
 	static void localTimeStampToUtc(ISC_TIMESTAMP_TZ& timeStampTz);
 
 	static void decodeTime(const ISC_TIME_TZ& timeTz, struct tm* times, int* fractions = NULL);
diff --git a/src/jrd/SysFunction.cpp b/src/jrd/SysFunction.cpp
index 43405b4f1e..47803b387d 100644
--- a/src/jrd/SysFunction.cpp
+++ b/src/jrd/SysFunction.cpp
@@ -538,8 +538,8 @@ void setParamsDateDiff(DataTypeUtilBase*, const SysFunction*, int argsCount, dsc
 	{
 		if (args[1]->isUnknown() && args[2]->isUnknown())
 		{
-			args[1]->makeTimestamp();
-			args[2]->makeTimestamp();
+			args[1]->makeTimestamp();	//// TODO:
+			args[2]->makeTimestamp();	//// TODO:
 		}
 		else if (args[1]->isUnknown())
 			*args[1] = *args[2];
@@ -2128,7 +2128,6 @@ dsc* evlDateAdd(thread_db* tdbb, const SysFunction* function, const NestValueArr
 }
 
 
-//// FIXME:
 dsc* evlDateDiff(thread_db* tdbb, const SysFunction* function, const NestValueArray& args,
 	impure_value* impure)
 {
@@ -2153,17 +2152,25 @@ dsc* evlDateDiff(thread_db* tdbb, const SysFunction* function, const NestValueAr
 	switch (value1Dsc->dsc_dtype)
 	{
 		case dtype_sql_time:
-			timestamp1.value().timestamp_time = *(GDS_TIME *) value1Dsc->dsc_address;
+		case dtype_sql_time_tz:
+			timestamp1.value().timestamp_time = *(GDS_TIME*) value1Dsc->dsc_address;
 			timestamp1.value().timestamp_date = 0;
+
+			if (value1Dsc->dsc_dtype == dtype_sql_time && value2Dsc->isDateTimeTz())
+				TimeZoneUtil::localTimeToUtc(timestamp1.value().timestamp_time, &EngineCallbacks::instance);
 			break;
 
 		case dtype_sql_date:
-			timestamp1.value().timestamp_date = *(GDS_DATE *) value1Dsc->dsc_address;
+			timestamp1.value().timestamp_date = *(GDS_DATE*) value1Dsc->dsc_address;
 			timestamp1.value().timestamp_time = 0;
 			break;
 
 		case dtype_timestamp:
-			timestamp1.value() = *(GDS_TIMESTAMP *) value1Dsc->dsc_address;
+		case dtype_timestamp_tz:
+			timestamp1.value() = *(GDS_TIMESTAMP*) value1Dsc->dsc_address;
+
+			if (value1Dsc->dsc_dtype == dtype_timestamp && value2Dsc->isDateTimeTz())
+				TimeZoneUtil::localTimeStampToUtc(timestamp1.value(), &EngineCallbacks::instance);
 			break;
 
 		default:
@@ -2178,17 +2185,25 @@ dsc* evlDateDiff(thread_db* tdbb, const SysFunction* function, const NestValueAr
 	switch (value2Dsc->dsc_dtype)
 	{
 		case dtype_sql_time:
-			timestamp2.value().timestamp_time = *(GDS_TIME *) value2Dsc->dsc_address;
+		case dtype_sql_time_tz:
+			timestamp2.value().timestamp_time = *(GDS_TIME*) value2Dsc->dsc_address;
 			timestamp2.value().timestamp_date = 0;
+
+			if (value2Dsc->dsc_dtype == dtype_sql_time && value1Dsc->isDateTimeTz())
+				TimeZoneUtil::localTimeToUtc(timestamp2.value().timestamp_time, &EngineCallbacks::instance);
 			break;
 
 		case dtype_sql_date:
-			timestamp2.value().timestamp_date = *(GDS_DATE *) value2Dsc->dsc_address;
+			timestamp2.value().timestamp_date = *(GDS_DATE*) value2Dsc->dsc_address;
 			timestamp2.value().timestamp_time = 0;
 			break;
 
 		case dtype_timestamp:
-			timestamp2.value() = *(GDS_TIMESTAMP *) value2Dsc->dsc_address;
+		case dtype_timestamp_tz:
+			timestamp2.value() = *(GDS_TIMESTAMP*) value2Dsc->dsc_address;
+
+			if (value2Dsc->dsc_dtype == dtype_timestamp && value1Dsc->isDateTimeTz())
+				TimeZoneUtil::localTimeStampToUtc(timestamp2.value(), &EngineCallbacks::instance);
 			break;
 
 		default:
@@ -2229,7 +2244,7 @@ dsc* evlDateDiff(thread_db* tdbb, const SysFunction* function, const NestValueAr
 		case blr_extract_month:
 		case blr_extract_day:
 		case blr_extract_week:
-			if (value1Dsc->dsc_dtype == dtype_sql_time || value2Dsc->dsc_dtype == dtype_sql_time)
+			if (value1Dsc->isTime() || value2Dsc->isTime())
 			{
 				status_exception::raise(Arg::Gds(isc_expression_eval_err) <<
 											Arg::Gds(isc_sysf_invalid_timediff) <<
@@ -2242,22 +2257,17 @@ dsc* evlDateDiff(thread_db* tdbb, const SysFunction* function, const NestValueAr
 		case blr_extract_second:
 		case blr_extract_millisecond:
 			{
-				//if (value1Dsc->dsc_dtype == dtype_sql_date || value2Dsc->dsc_dtype == dtype_sql_date)
-				//	status_exception::raise(Arg::Gds(isc_expression_eval_err));
-
 				// ASF: also throw error if one value is TIMESTAMP and the other is TIME
 				// CVC: Or if one value is DATE and the other is TIME.
-				const int type1 = value1Dsc->dsc_dtype;
-				const int type2 = value2Dsc->dsc_dtype;
-				if ((type1 == dtype_timestamp && type2 == dtype_sql_time) ||
-					(type1 == dtype_sql_time && type2 == dtype_timestamp))
+				if ((value1Dsc->isTimeStamp() && value2Dsc->isTime()) ||
+					(value1Dsc->isTime() && value2Dsc->isTimeStamp()))
 				{
 					status_exception::raise(Arg::Gds(isc_expression_eval_err) <<
 												Arg::Gds(isc_sysf_invalid_tstamptimediff) <<
 													Arg::Str(function->name));
 				}
-				if ((type1 == dtype_sql_date && type2 == dtype_sql_time) ||
-					(type1 == dtype_sql_time && type2 == dtype_sql_date))
+				if ((value1Dsc->dsc_dtype == dtype_sql_date && value2Dsc->isTime()) ||
+					(value1Dsc->isTime() && value2Dsc->dsc_dtype == dtype_sql_date))
 				{
 					status_exception::raise(Arg::Gds(isc_expression_eval_err) <<
 												Arg::Gds(isc_sysf_invalid_datetimediff) <<