When a new credit adjustment is created under
Top > Billing Director > Receivable > Documents> Adjustments > New Credit Adjustment it is possible to choose any date in the Document date field, but OBAS does not save it.
After Credit Adjustment is created the Document date is set to the current date.
Such behavior was classified as the software issue PBAS-30067 was fixed in OBAS 4.5.5.
To resolve the issue, please upgrade to 4.5.5 as described in How to upgrade OBAS system to the latest available version.