How to compare the gl_periods table Period_name column with dates

Hello,

How to compare the Period_name column in gl_periods table with dates, When we are comparing we getting the "ORA-01843: not a valid month" error.

Query :

select

upper(period_name)

,to_date(period_name,'MON-RR')

,gp.*

from gl_periods gp

where 1=1

and period_name not like 'Adj%'

and to_date(period_name,'MON-RR') = to_date('01-JAN-1996')

;

ERROR :

ORA-01843: not a valid month

01843. 00000 -  "not a valid month"

*Cause:  

*Action:

Thanks,