I'm self employed and had private health insurance for 2017 (premiums entered on Sch C). Where to enter a premium paid in Dec 17 for a Marketplace plan effective Jan 18?

If your business (Schedule C) uses the cash method and you paid your health insurance premium in 2017, enter it on your 2017 Schedule C.

If your business (Schedule C) uses the accrual method and you paid your health insurance premium in 2017, enter it on your 2018 Schedule C.

The cash method .  The cash method is the more commonly used method of accounting in small business. Under the cash method, income is not counted until cash (or a check) is actually received, and expenses are not counted until they are actually paid.

The accrual method.  Income is counted when the sale occurs, and expenses are counted when you receive the goods or services. You don't have to wait until you see the money, or actually pay money out of your checking account, to record a transaction. 

See IRS Publication 334 Accounting Periods and Methods for additional information.

Thanks. That's what I was going to do, except it specifically says not to enter premiums paid through the Marketplace on Schedule C....