@Jakob ********
most offices are now requesting the "standard bank letter" (the one banks have been issuing all along) AND a year print out of the transaction details (because most people don't update their bank books often enough to get every transaction printed in the book, just a summary of money in - money out) AND updating the bank book on the date you apply.
MOST offices (Bangkok, Chiang Mai, Phuket, allow the standard bank letter to be up to 7 days old, there's no rule on the year transaction detail and the bank book has to be updated the date you apply. Your office may vary, CHECK WITH THEM FIRST.