DECLARE EXTERNAL FUNCTION XD_TIMEBETWEEN
    CSTRING(32),
    TIMESTAMP,
    TIMESTAMP
RETURNS INTEGER BY VALUE
ENTRY_POINT '__TimeBetween' MODULE_NAME 'xdUDF';

Function __TimeBetween(AFormat :PAnsiChar; AStartDate :PISC_QUAD; AEndDate :PISC_QUAD): Integer;
Var tmStartDate  :TCTimeStructure;
    tmEndDate    :TCTimeStructure;
Begin

  isc_decode_date(AStartDate, @tmStartDate);
  isc_decode_date(AEndDate,   @tmEndDate);

  // тут всё при дебаге убрал ...
  Result     := 123; 

End;

select XD_TIMEBETWEEN('H', CURRENT_TIMESTAMP-5566, CURRENT_TIMESTAMP)
from rdb$database