The IRS has extended due dates for filing returns that were originally extended to either 9/15/17 or 10/15/17 to 1/31/18. Estimated payments due 9/15 are also extended.

The page for Harvey lists affected counties and all the details:

The page for Irma, as of my post, is missing details. I would expect the details to be added later.

This page lists the affected areas for Irma. If you should be on the list, but are not, wait a few days and re-check the link, It took the IRS about a week after Harvey to finalize the list.