Hello and thanks for trusting me to help you today. I am a tax adviser with over 20 years of experience.
The first payment in April was for Jan through march.
1st payment: April 15, 2015
2nd payment: June 15, 2015
3rd payment: September 15, 2015
4th payment: January 15, 2016
The first quarter is three months (January 1 to March 31), but the second "quarter" is two months long (April 1 to May 31), the third is three months (June 1 to August 31) and the fourth covers the final four months of the year.
Quickbooks appears to be estimating your payments based on the true 3 calendar months in the quarter.
At years end your accounting
for the taxes paid and the total liability
will be shown.