On May 5, John Plumbing receives a phone call from a custome…

On May 5, John Plumbing receives a phone call from a customer needing a new water heater and schedules a consultation for a free estimate on May 7.  On May 8, John Plumbing installs the new water heater. The customer pays for the service on May 12. According to the revenue recognition principle, on which date should John Plumbing record service revenue?