YTD => Year to Date, right?

So if you're in January you want the whole of the previous year?

If so, you can find the start date by:

- Subtracting one month from the current date
- Returning the start of the year for the corresponding date with the 'y' mask of trunc

And the end date as:

- If the current month = January, return the start of the year (assuming non-exclusive boundary; i.e. all dates strictly less than the end)
- Otherwise current date

alter system set fixed_date = '2018-06-02';

select trunc ( add_months ( sysdate, -1 ), 'y' ) st,
       case
         when extract ( month from sysdate ) = 1 then
           trunc ( sysdate, 'y' )
         else
           sysdate
       end en
from   dual;

ST                     EN                     
01-JAN-2018 00:00:00   02-JUN-2018 00:00:00  

alter system set fixed_date = '2018-02-01';

select trunc ( add_months ( sysdate, -1 ), 'y' ) st,
       case
         when extract ( month from sysdate ) = 1 then
           trunc ( sysdate, 'y' )
         else
           sysdate
       end en
from   dual;

ST                     EN                     
01-JAN-2018 00:00:00   01-FEB-2018 00:00:00 

alter system set fixed_date = '2018-12-20';

select trunc ( add_months ( sysdate, -1 ), 'y' ) st,
       case
         when extract ( month from sysdate ) = 1 then
           trunc ( sysdate, 'y' )
         else
           sysdate
       end en
from   dual;

ST                     EN                     
01-JAN-2018 00:00:00   20-DEC-2018 00:00:00  

alter system set fixed_date = '2019-01-10';

select trunc ( add_months ( sysdate, -1 ), 'y' ) st,
       case
         when extract ( month from sysdate ) = 1 then
           trunc ( sysdate, 'y' )
         else
           sysdate
       end en
from   dual;

ST                     EN                     
01-JAN-2018 00:00:00   01-JAN-2019 00:00:00   

alter system set fixed_date = none;