It's the Paper, Stupid --Dickeson

Paper won't leave WIP until the job is invoiced. The greater the invoice-time, the less the profit. (We're not making a nickel until it's billed.)

Follow the paper into accounts receivable. As long as it's in a receivable you still haven't captured that elusive nickel. Note the date and time the job becomes part of a receivable. Next, note the date and time payment is received. The difference between billing and collecting is the receivable dwell-time.