Oracle cannot find date for 1800's year

I know this is stupid but I have to ask the date problem. I am dealing with a date field which value frange from 1800 to now. I do the following code to change the date string 'YYYYMMDD' into a date:

v_priority_date := TO_DATE(substr(priority, 1, 8), 'YYYYMMDD');
v_priority_date_text := TO_CHAR(v_priority_date, 'DD-MON-YYYY');
v_priority_date := TO_DATE(v_priority_date_text, 'DD-MON-YYYY');
v_priority_date_text := TO_CHAR(v_priority_date, 'DD-MON-YY');
v_priority_date := TO_DATE(v_priority_date_text, 'DD-MON-YY');

Then I use the follwing select statement to search for the record:

        SELECT water_allocation_id
          INTO v_water_allocation_id
          FROM water_alloc_priorities
         WHERE priority_number_date = v_priority_date
           AND priority_number = v_priority_number;

It comes to my notice that some of records in the range 1889 - 1899 cannot be pciked up the query. What's wrong?

I tried to use the compare statement like:

   WHERE TO_CHAR( priority_number_date, 'YYYYMMDD') = TO_CHAR( v_priority_date, 'YYYYMMDD')

But it took 9 hours to run the job as Oracle will fo a full table scan for each record is selected.

Please help.

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Best regards,