March 15 to be effective for the current tax year.

There's another tax issue relating to filing an S-election. If you're a corporation taxed as a C corp, and you convert to an S Corp tax structure, there is something called Built In Gain (BIG) tax that phases out over time. This relates primarily to a sale of the corporation and its assets. For most small businesses who are thinking about making an S-election, they're previously taxed as a sole proprietorship or a partnership, so there is not BIG tax issues. And if you don't sell the business for 5 years (used to be 10 years), it's not an issue either for a C corp making the election.