I would describe it like this. Let's say you receive $10,000 in March for a service that will not be completed until June. You started the job in May and finished 50%. So your revenue for March and April would be zero (even though you received the 10k in March). Your revenue for May would be $5,000 (since 50% of the job is completed) and the other $5,000 would be your June revenue (assuming completion).