If you change the use of your vacation home from personal use to rental use at any time other than the beginning of the year, you must divide yearly expenses, such as depreciation, taxes, and insurance, between personal use and rental use. You can deduct as rental expenses only the portion of total expenses allocable to the portion of the year the property was used or held for rental.
The parents reduced their taxable income by $1,000 by using a partial-year allocation of their vacation home expenses. Since the parents income was taxed at the 30% tax bracket, the income reduction may decrease their income taxes by $300 ($1,000 x 30%).
If you are in a combined (federal, self-employment, and state) tax bracket of 30%, you could reduce your taxes up to $300 for every $1,000 of income reduction by using a partial-year allocation of your vacation home expenses.