I am not using the "how often" selection for the reminder to "to pay estimated taxes" . It's true that using this selection gives you the due dates. However, the IRS let's you pay ahead of time and it is convenient for me to pay in December rather than waiting 2 weeks until Jan 15. So I set up quarterly payments. In my original post the tax planner picked up a payment scheduled for March 31, 2020 and treated it as a 2019 payment.
Finally I would say that this used to work correctly in Quicken 2016 before I switched over to the subscription service. Quicken shouldn't assume that we are all going to wait until the 11th hour to send in our tax payments.
From C. D. Bales:
"In Tax Planner, under Estimated Tax Payments, the total for Scheduled Bills and Deposits includes a scheduled payment in 2020. This payment would be for2020 estimated taxes, not 2019."
I am not able to reproduce your problem.
I created a Reminder for estimated tax payments starting with April 15, 2019 (the first estimated tax payment for tax year 2019). I manually entered an estimated tax payment dated January 15, 2019. That is my only actual transaction for estimated taxes.
In my test, Quicken correctly did not show the January 15, 2019 estimated tax payment in the Tax Planner for 2019.
Estimated tax payments are for the preceding quarter. So:A January 15, 2019 estimated tax payment is for 2018 estimated taxes.An April 15, 2019 estimated tax payment is for 2019 estimated taxes (it is the first estimated tax payment for 2019).A January 15, 2020 estimated tax payment is for 2019 estimated taxes.