We need to stop update of sales order on 12pm day prior to Scheduled Ship date

We have a requirement where we need to stop update of sales order & order lines by users on 12pm day prior to planned shipment i.e. Scheduled Ship date.
We have created a processing constraint & it allows restriction based on Validation rule set>Attribute name = 'Scheduled Ship Date' and in the value string we are able to enter only the SYSDATE. But we want to use function as mentioned above. Is there any possibility to achieve this feature.