ORA-01843: not a valid month tips Oracle Error Tips by Donald Burleson(S. Karam)

The Oracle docs note this on the ora-01843 error:

Resolving ORA-01843 includes bad data issues.  The reason a ORA-01843 occurs includes these reasons

• Using an incorrect NLS_DATE_FORMAT
  This happens a lot when you see data coming from US
  (where the Month is normally written before the day)
  but the format in the application is still the "European" style.
  (where the day is written before the month).

• Using a incorrect NLS_DATE_FORMAT:
  Such as trying to insert a "written" month name but are using a "numeric" month in the mask.

An "ORA-01843: not a valid month" error is not a "oracle problem" but most likely caused by not defining proper date masks in your code or relying on incorrectly assumed defaults."

Also important is to keep in mind that ORA-01843 is not always thrown, even when there is incorrect data.

(You may want to view MOSC (Doc ID: Note: 338832.1) documentation for full information regarding ORA-01843 debugging.)