Received CP30A for one client; discrepancies are as follows:

1st installment - IRS penalty calc appears to be accurate. 

2nd installment - IRS used 6/15/2023 to 6/29/2023 (14 days), whereas client had an underpayment from 6/15  to 9/15 (92 days).

3rd installment - IRS did not assess a penalty, whereas client appears to have an underpayment.

4th installment - IRS used 2/15/2024, as BobKamman suggested.