VSys can automatically credit volunteer assignments with hours after their shifts are done. Filter the assignments by Job, Location, Lists (for the volunteer), Groups, People types and Volunteer types. Optionally limit by assignment duration and only if the scheduled assignment's hours wouldn't result in hours that conflict with existing credited hours records.
You'd normally use this tool only for your most reliable volunteers: it gives credit for assignments with no knowledge of whether or not the volunteer actually showed up for his shift!