Yes, it depends on the accounting method you use. Most taxpayers are cash
method, meaning that the income and deductions are counted once received or paid. So only the expenses actually paid in 2015 would be included.
However, if you
have elected the accrual method,
then the income and deductions are reported when earned or incurred, so a payment in January 2016 for an expense incurred in 2015 would be allowed.
See the FAQ below for more info. https://ttlc.intuit.com/replies/3301956