It is possible, when creating extra fields for a client to have those include an effective date.
To enable this, at the admin level of the company (before you login as SA), find the available Functionality widget. If you don't see it on your screen, you can add it by going to edit tabs at the top.
Once you find the widget, use the search function to find the Timesheet Effective Dates option.
By enabling this, it gives you effective dating on both the extra fields items, as well as the root cost centers.