Date of check. Unlike actual receipt, constructive receipt does not require physical possession of the item of income in question.
Per IRS Constructive receipt, Publication 538 - Internal Revenue Service Accounting Periods and Methods:
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.
Note: 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.
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