You can do this using expressions in n8n. Here’s an example workflow that uses a Set node to stimulate the data coming from your Webhook node and another Set node that removes two days from the date.
Instead of expressions, you can also use the Function node for a better implementation than the one shown in the example above. If you are running a self-hosted version of n8n, you can also use built-in and external modules for easier manipulation of data.
You can find some handy JS snippets to use with n8n here. Hope that helps!