No, I wasn't referring to the 'Created Date' column.
I was referring to fields in the InfoPath form itself that are formatted as date or date/time fields. When promoting your fields, you cannot 'allow users to edit data in this field by using a datasheet or properties page’ on any date formatted field. What this means is that a workflow (we use Nintex) will not see that date field and so it can't update it.
I made a very interesting discovery though in troubleshooting all of this and it resolves the issue. If you make your field plain text and include the word 'date' in the field name, when you promote the field to your SharePoint library, SharePoint automatically creates the column as a date/time column. So now your field (column) is editable by the workflow (provided you selected the 'allow users to edit data in this field by using a datasheet or properties page' when promoting it) and even better, (for me anyway) the column is then able to be made editable and is then able to be edited or deleted in SharePoint. The ability to delete is fantastic if you end up with the problem where your form has somehow lost it's connection to the column. You can then delete it and republish your form and recreate the column without having to rename it in the property promotion and change workflows etc.
I tested this several times after discovering it and verified that it worked every time I created a plain text field with the word 'date' as part of the field name.