@revjrblack, how is your form set up? You should only need the date range field, not the date range field and the start and end date fields because the start and end date fields are part of the date range field.
For updating an existing record, use the same link as above, but also use this link to generate the dynamic data from the database, Using Date Range Picker with Dynamic Values
Are you using the Equal To validation to confirm the start and end dates are equal to the date ranger picker? Trying to understand your work flow for entering them in twice.
To answer your question about the error, the only thing I can think of is that the $_POST variables are wrong somehow.
Ok, let’s look at your server action. Do you have the start and end dates listed in the $_POST variables and in the insert statement? I know it might sound like a silly question, but I have found that most of my errors are from something very small that I overlooked.