Renting under fair market value (FMV) to a non-relative...

I bought a rental last June and am renting it at less than the fair rental price.

If it is less than FMV, do I enter the approximately 180 days (not at a fair price) in the section title "Days rented at a fair rental price"?

Secondly, if I enter the rental income later, how do I acocunt for the lost revenue by renting under market value?
    Sorry, wishful  but incorrect thinking if you are assuming you can take a tax break for renting below market value.  There is no accounting for lost revenue.  You simply made less income.  

    HOWEVER, the tax consequence of renting below market value is that you are not allowed to claim a loss if expenses exceed income.  If you are renting substantially below FMV, you must follow these tax rules

    From IRS pub 527
    Not Rented for Profit
    If you do not rent your property to make a profit, you can deduct your rental expenses only up to the amount of your rental income. You cannot deduct a loss or carry forward to the next year any rental expenses that are more than your rental income for the year.

    Where to report. Report your not-for-profit rental income on Form 1040 or 1040NR, line 21. For example, if you are filing Form 1040, you can include your mortgage interest and any qualified mortgage insurance premiums (if you use the property as your main home or second home), real estate taxes, and casualty losses on the appropriate lines of Schedule A if you itemize your deductions (or report your state or local real estate taxes on Schedule L if you do not itemize your deductions).

    Claim your other rental expenses, subject to the rules explained in chapter 1 of Publication 535, as miscellaneous itemized deductions on Schedule A (Form 1040), line 23, or Schedule A (Form 1040NR), line 11. You can deduct these expenses only if they, together with certain other miscellaneous itemized deductions, total more than 2% of your adjusted gross income.
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