As an engineer, I would not communicate a delay in a project to an important client at all. However, supposing it were my responsibility to communicate a delay, (say, I'm a project manager or some such), I would be compiling velocity metrics which, early on, suggest a completion date beyond what is anticipated. If after reviewing with upper mgmt we feel that we cannot bring that forward with extra effort, we assess reasons for the delays and communicate the delay, with supporting metrics and reasons, as early and as transparently as possible to the client. If the client is important enough, it may be that upper mgmt wants to handle the communication at higher levels and/or pull in more resources in order to meet the deadline after all. In short, as an engineer, it's above my pay grade to communicate this sort of thing to an important client. Even the project manager would require higher level buy-in and approval before potentially losing an important client.