What is the impact on a running database when OS date is changed and set to past My SAP back end db is db2. Need to understand the impact on this running database (db2 under HPUX) if I change the OS date and set it to a past date please.

Thanks.
That's an extremeley broad question.  If your DB's tables use date in the primary keys you could run into duplicate problems.  If you query by date range you may have duplicates or holes in your data.  You may even have difficulty saying which row was written before another, depending on your design.  Reproducing any previous state may become impossible e.g. "What was the customer's balance on the 1st January?"  and so forth.

Really, you need to understand your application and DB design deeply, and test, test, test.