@Vladimir *******
then you can get the bank letter as long as it's within 7 days from the date you apply for the extension
SO if you're applying for the extension on Mar 1 you can get the letter any time after Feb 22,
They'll do it in the basement at the B/B branch at Chaengwattana Building B no issues at all if you want to get it the day you apply (you need to update your bank book anyway)