US SB684

Extends the deadline for S corporation elections from 2½ months into the tax year to the due date of the S corporation return (including extensions), allowing businesses more time to elect pass-through tax status

Applies similar extended election timing rules to qualified subchapter S subsidiaries and qualified subchapter S trusts

Changes individual estimated tax payment due dates from June 15 and September 15 to July 15 and October 15, providing taxpayers an additional month for the second and third quarterly installments

Extends the "mailbox rule" (which treats timely-mailed documents as timely filed) to electronic submissions and payments, so the transmission date rather than receipt date determines timeliness

Grants the Secretary authority to treat late S corporation revocations as timely if reasonable cause existed for the delay