"You must deposit [SEP IRA] contributions for a
year by the due date (including extensions) for filing your federal income tax
return for the year. If you obtain an extension for filing your tax return, you
have until the end of that extension period to deposit the contribution,
regardless of when you actually file the return."
In TurboTax, note that the instruction on the screen
"[name] Contributions" are to "Enter the amounts that [name]
contributed or will contribute to a Keogh, SEP, or Simple Plan for 2017".
This includes amounts actually contributed in 2018 before the due date of the
return. Note that this screen can be found under "Less Common Business
NOTE: when making this contribution, you MUST tell the plan
administrator that the contribution is for 2017, because their default will be
to make it for 2018.