Brian,
This is a common problem. Here is a solution for you using a reverse relationship.
Make table Batch Date. Allow users to create records there but not edit anything.
Create a Summary field between the Lab Test and Batch Date field to give you the Minimum value of the Record ID# in the Batch Date table. By definition, that is the first one created.
Create a new Parent Child relationship between the Batch Date and Lab Test with Batch Date as the Parent. Make sure you use the Min RID Batch as the reference field when you create the relation ship. Do not let it create a new field.
Now in the new relationship make Date a look up field in the Lab Test table. Change the name to something like First Date/ Original Date what ever makes sense for your lab. You can now use that Date in all your forms and reports and it will not change.
You can also learn more about that Date, such as who put it in, [Record Owner], the date and time it was set [Date Created] and so on.
