Individuals file 2017 returns (Form 1040, Form 1040A or Form 1040EZ); alternatively, file for an automatic six-month extension (Form 4868, Application for Automatic Extension to File U.S. Individual Income Tax Return).
Calendar-year C corporations file 2017 returns on Form 1120, U.S. Corporation Income Tax Return; alternatively, file for an automatic five-month extension using Form 7004, Application for Automatic Extension of Time to File Certain Business Income Tax, Information, and Other Returns.
Individuals and calendar-year corporations pay the first installment of 2018 estimated tax.
Monthly depositors deposit FICA and withheld income tax for March.