ORA-01843: not a valid month

[slimdave]

I wouldn't rely on alter session. Just specify the date format:

NVL(TRUNC(BeginDate), to_date('01.01.1800','DD.MM.YYYY')) <= TRUNC(SYSDATE)

or

NVL(TRUNC(BeginDate), date '1800-01-01') <= TRUNC(SYSDATE)