I prefer to include Billable Time as income because it is important when analysing the financial performance of a business for a specified period to take into account its production as represented by Billable Time. If the unbilled time is not taxable until is invoiced you can enter a reconciling adjustment on a tax return or at the end of the financial year transfer the balance of Work in Progress from the income account to a liability account named Deferred Income or Unearned Income.