At this time we don't have a way to remove old hours without deleting the volunteers permanently, so they will be archived.
You can, however, set a new start date and all charts and the volunteer list hours will reflect that date.
To set a new start date:
1) Go to the Organizer Dashboard >> Settings >> More Settings
2) Select Other Configurations: Set your new Start Date (choose the month and date from the menu)
You can also set a custom milestone to set up a specific time frame that your volunteers have to meet and when that date is met the time frame resets and the volunteers can have a clear time sheet and goal.
To set a custom milestone time frame:
1) Organizer Dashboard >> Settings
2) Milestones >> Over what kind of time frame? select Custom.
3) Set your frequency and start date. This method will have a frame of time over which your volunteers will be required to submit their hours by. More info on this in this link to the update.