Start and End Dates columns on customers and vendor screens allow you to enable or disable them on transaction screens. If the transaction date does not fall between the start and end date of a particular customer, SL will not allow you to select that customer for the transaction.
When you add a new customer (or vendor), sql-ledger defaults start date to the current date and end date is kept blank. If you want to allow this customer for any transaction date, keep both start and end date blank.
This feature causes confusion if you add a new customer today, accepting the default for start date, and try to post an invoice in any past date. You will be unable to select customer on invoice screen. The trick is of course to specify the correct start date (the one which matches the invoice date in the past).
Start and end dates columns do not affect any reports.