May I suggest you seek out training or a good instructional manual? Based on what I saw of your posts, I believe it will greatly speed up your development and save you money (in lost time) in the long run.
Getting extra help learing the system is a good idea.
In the meantime...
"Then I create a new layout and put fields from several of the tables on it."
Likely explains your trouble. Each layout refers to one table (technically a table Occurrence) directly and then, if you add fields from them, links to other tables via the relationships you create in Manage | Database | Relationships
Likely, your date field comes from a different table than your layout's table and the relationship is not allowing you to enter data. You may need to enter data in another field that is used in your relationship to link the two tables. You may need to enable "Allow creation of records via this relationship" for the table where you defined your date field.