The Year Ending Date is the last day of a 12-month period (called a Metric Year) which a set of metrics are based on. A Metric Year starts on the first day of one month, and ends 12 months later on the last day of the last month (ex: 6/1/2015 – 5/31/2016). There are 12 possible Year Ending Dates in each calendar year (Jan 31, Feb 28, Mar 31, etc).
All Metrics in Portfolio Manager are calculated based on 12 full calendar months of data. The 12-month period associated with a given metric is often represented by the Year Ending Date (the last day of the 12-month period). For example, if your Year Ending Date is 12/31/2016, your metrics would be based on the calendar year 2016.