SELECT CAST('00:00:00 Europe/Moscow' AS TIME WITH TIME ZONE),
       CAST('00:00:00 +3' AS TIME WITH TIME ZONE) FROM RDB$DATABASE

SQL> connect 'inet4://localhost:3054/test' user SYSDBA password 'masterkey';
Database: 'inet4://localhost:3054/test', User: SYSDBA
SQL> SELECT CAST('00:00:00 Europe/Moscow' AS TIME WITH TIME ZONE),
CON>        CAST('00:00:00 +3' AS TIME WITH TIME ZONE) FROM RDB$DATABASE;

                                          CAST                                           CAST
============================================== ==============================================
00:00:00.0000 Europe/Moscow                    00:00:00.0000 +03:00

SQL>

void UtilInterface::decodeTime(ISC_TIME time,
	unsigned* hours, unsigned* minutes, unsigned* seconds, unsigned* fractions)
{
	tm times;
	isc_decode_sql_time(&time, &times);

	if (hours)
		*hours = times.tm_hour;
	if (minutes)
		*minutes = times.tm_min;
	if (seconds)
		*seconds = times.tm_sec;
	if (fractions)
		*fractions = time % ISC_TIME_SECONDS_PRECISION;
}

void UtilInterface::decodeTimeTz(CheckStatusWrapper* status, const ISC_TIME_TZ* timeTz,
	unsigned* hours, unsigned* minutes, unsigned* seconds, unsigned* fractions,
	unsigned timeZoneBufferLength, char* timeZoneBuffer)
{
	try
	{
		tm times;
		int intFractions;
		TimeZoneUtil::decodeTime(*timeTz, CVT_commonCallbacks, &times, &intFractions);

		if (hours)
			*hours = times.tm_hour;

		if (minutes)
			*minutes = times.tm_min;

		if (seconds)
			*seconds = times.tm_sec;

		if (fractions)
			*fractions = (unsigned) intFractions;

		if (timeZoneBuffer)
			TimeZoneUtil::format(timeZoneBuffer, timeZoneBufferLength, timeTz->time_zone);
	}
	catch (const Exception& ex)
	{
		ex.stuffException(status);
	}
}

select current_time from rdb$database

Error Message:
----------------------------------------
Access violation at address 6AEF917F in module 'fbclient.dll'. Read of address 00000004

[6AEF917F] isc_blob_set_desc + $465F
[00403BDF] System.@HandleAnyException + $33
[004F5BDE] Firebird.IUtil.decodeTimeTz (Line 253, "Firebird.pas" + 1) + $1E
[005345EC] FIBQuery.TFIBXSQLVAR.GetAsDateTime (Line 821, "FIBQuery.pas" + 30) + $30
[0052B8CC] FIBDataSet.TFIBCustomDataSet.FetchCurrentRecordToBuffer (Line 2276, "FIBDataSet.pas" + 69) + $E
[0052EBE7] FIBDataSet.TFIBCustomDataSet.GetRecord (Line 3991, "FIBDataSet.pas" + 50) + $16
[0050E152] Db.TDataSet.GetNextRecord (Line 9185, "DB.pas" + 9) + $13

SELECT
    CAST('12.12.2018 00:00:00 +4:00' AS TIMESTAMP WITH TIME ZONE)
   -- , current_timestamp as v
FROM
    RDB$DATABASE

SELECT
    CAST('12.12.2018 00:00:00 Africa/Asmera' AS TIMESTAMP WITH TIME ZONE)
FROM
    RDB$DATABASE