"No more than 2 months and 15 days after the beginning of the tax year the election is to take effect, or At any time during the tax year preceding the tax year it is to take effect."
From the S-election Form 2553 instructions. So you would have had to do it on March 15th this year if you started on January 1. It would probably be less of a headache to elect it for next year, since if you did so now you would have a short year.
Alternatively, if you find a good tax accountant, they can usually help you with a late-filed election. There is special verbiage that lets you file late and have it be effective when you want it to, i.e. the whole year for 2018.