If your business operates on a cash basis (and it probably does), then income is governed by a doctrine known as constructive receipt. From IRS Pub 538:
Constructive receipt. Income is constructively received when an amount is credited to your account or made available to you without restriction. You do not need to have possession of it. If you authorize someone to be your agent and receive income for you, you are considered to have received it when your agent receives it. Income is not constructively received if your control of its receipt is subject to substantial restrictions or limitations....
You cannot hold checks or postpone taking possession of similar property from one tax year to another to postpone paying tax on the income. You must report the income in the year the property is received or made available to you without restriction.
In the case of a check, you have constructive receipt of the check when you receive it, not when you cash it or deposit it. In your example, the income would belong to tax year 2017.