The 2019 over-recognition may cancel out the 2018 under-recognition but lookback is only done when the contract is complete.
Client’s actual 2018 TI (Taxable Income) was $225k. That amount includes the $100k recognized in 2018 with respect to Contract #1 (that should have been recognized in 2017 because actual differed from estimates). Client’s actual 2018 tax was, of course, computed on the $225k of TI.
Now pretend 2019 just ended, the year in which Contract #2 was completed. As it turned out, $100k of Contract #2 should have been recognized in 2018. So, when we do the lookback for “Filing Year” 2019, and we look back to 2018, we need to compute the increase to the 2018 tax that results from the $100k associated with the Contract #2 lookback.
Are you saying that when we do just that, we compute the tax differential between (1) 2018 actual TI of $225k and (2) 2018 redetermined TI of $325k…despite the fact that 2018 actual TI was inflated by the $100k associated with Contract #1 that “should have been” recognized in 2017?
For 2019, he'll pay interest on the tax on the $100k that he'll report in 2019 instead of 2018.
How do we compute the tax on that $100k of Contract #2 income?
Don’t we do this: Look at the 2018 tax return, as filed, and jot down the gross tax liability. Then, we hypothetically add $100k to 2018’s existing/actual Taxable Income and compute the tax on that higher amount. The difference between the actual tax (showing on the 2018 return as filed) and what that tax would have been had 2018 taxable income been $100k higher is the “tax increase.” Then we apply an interest rate to it.
Does that make sense?
One might argue that this piece:
Look at the 2018 tax return, as filed, and jot down the gross tax liability.
…involves inflated numbers. Remember, the actual TI for 2018 included $100k that “should have been” reported in 2017.
If we pretend the 2018 actual TI, as filed, was $225k…then which is the correct calculation to determine the 2019-to-2018 lookback:
Is it Option #1 (described above)…$225k actual TI for 2018 vs. $325k redetermined TI for 2018…or…
Is it Option #2…$125k “actual” TI [since the $225k real actual TI was inflated by $100k b/c of the 2018-to-2017 lookback] vs. $225k redetermined for 2018?
Either way, the delta is $100k. We are, as you say, computing the tax increase associated with the $100k Contract #2 income that we reported in 2019, but “should have reported” in 2018. But it matters a great deal what that $100k “is added to” (i.e. the base/benchmark) in computing the tax on the base and the tax on the “Base plus $100k.”