That does help a little. I wasn’t sure what column type you were talking about originally. The “Date” column is user-editable whereas, you are correct, the “Created Time” field is not.
As to your question, you’re also correct in that it should reflect the time and date when you click the little ‘+’ icon to add a new record. My only guess as to why things may display incorrectly for the first 18 rows is you merely deleted the data (i.e., highlighted everything and then delete) as opposed to deleting the actual records (i.e., selecting a row via the checkbox on the far left, right-click, and delete record). This seems to be the case since, as you say, new records you add are behaving correctly.
If this sounds correct, you could “fix” the data you currently have by adding 18 new rows, copying and pasting the information, and then deleting the records (by the checkbox/delete method, not the keyboard delete). To have a little more control over the column you could change it to a “Last modified time” field, but that might not be helpful in your use case here.