Oracel Apex automation error: ORA-01843: not a valid month

I am the developer of ABC system in Apex for the our office. It uses PL/sql, and

i'm new to oracle apex. I am only learning on how to use automation tool to schedule jobs to update data.

For the func: PRRStatus_upd_Step2 to update the field "Status" in tbl_PRR, it works well when i manually run it.

However, when i scheduled it in oracle apex automation, if it autorun at 600pm, it will sometimes fail.

I tested, it will fail at 335pm as well sometimes. The error: ORA-01843: not a valid month

Inside the function PRRStatus_upd_Step2, it is just the code below: (and WorkingDays10_Open_FINAL is another func in the apex system)