Q1 is the first quarter of the year, which is January 1st through March 31st.
Q2 is the second quarter of the year, which is April 1st through June 30th.
Q3 is the third quarter of the year, which is July 1st through September 30th.
Q4 is the fourth and last quarter of the year, which is October 1st through December 31st.
A quarter is every 3 months in a calendar year starting with January.
Quarter to date is the time period from the start of the current quarter to the current date.
Quarterly means every 3 months or every quarter.