The "Tax Administration Simplification Act" proposes significant changes to federal tax law, primarily focusing on S corporation administration. It extends the deadline for a small business corporation to make an S corporation election from the 15th day of the third month of the taxable year to the due date for filing the S corporation's return , including extensions. This bill also grants the Secretary of the Treasury authority to treat a late S corporation revocation as timely if there was reasonable cause for the delay, providing greater flexibility for businesses. Beyond S corporation rules, the legislation modifies the schedule for individual estimated income tax payments, shifting the second and third quarterly installment due dates from June 15 to July 15 and from September 15 to October 15 , respectively. Crucially, it extends the long-standing "mailbox rule" to electronic submissions and payments, meaning the date a document or payment is electronically transmitted will be considered the filing or payment date, even if received later. This aims to modernize tax administration and reduce penalties for timely electronic transmissions.