I suspect you are not talking about invoices received, but rather payments received. Is that correct?
Your situation goes to the differences between cash and accrual accounting. As a result, what you report your accountant to have done seems questionable. If you are using accrual accounting, s/he might have moved income recognized to 2015 even though you were not paid until 2016 because that is when the income was earned. In accrual accounting, income is not dependent on when payment is received, but when earned.
If you are using cash accounting, you would certainly not move payments received to the year when work was performed, because cash accounting reports based on when payment is received.
So either your terminology is being misused, you misunderstood exactly what your accountant said, or your accountant actually got things wrong. We need more detail to know.
And yes, changing dates on either invoices or payments is wrong and may actually be illegal. A tax auditor might suspect fraud through attempted misstatement of income.
Manager can handle either cash or accrual accounting, but you have to understand which is which and which you are using.