In Configuration > General, there is an option to establish the start date (mm/dd) of the volunteer year. The 'Total for Volunteer Year as of Start Date' calculates the number of hours between the volunteer start date this year and the start date of the report.

For example:

A volunteer year for an organization runs January to December. This is specified at Config > General by entering 01/01 in the Start Date of the Volunteer Year field:

Volunteer Robert Hernandez has 2 time sheets on his Volunteer tab:
Photographer 02/01/18 for 15 hours
Crowd Control 10/12/18 for 11 hours

Let's run the Time Sheet Report for the last quarter of 2018. This would be done by setting a specific date range of 10/01/18 - 12/31/18 on the General tab.

The Total as of Start Date shows 15 hours as the Photographer time sheet instance happens between 01/01/18 (volunteer year start date for this year) and the start date of the report 10/01/18