One of the most important aspects of being a writer staring at FADE IN: of a commission is making sure you deliver, not just on time but also exactly what you've been asked to. Failure is not an option.
If you promise to deliver a screenplay by a certain time always try to finish it and hand it in early. It makes you look good. I alway like to use the example of Scotty from Star Trek, who always gives an over estimated time for the repairs to be completed, so he can finish them earlier and maintain his reputation as a miracle worker. The same should apply to you.
When I was asked recently to make significant changes to a screenplay at the last minute, roughly about a third of what was written, because of notes the director had received from the money men, I promised him it would be finished in no more than four days. I actually worked my butt off to deliver a rough draft by the end of the very same day. The thing is I knew the director wanted the changes fast, I knew they were big, and I knew they were necessary, so I cleared my desk and got my head down and got it done in a day. I could have taken my time, handed it in, in the four days I had promised, but I wanted to get it done and do a good job. A final check of the script the same night by the director and I was given the go ahead to tidy it up and proof read it the next day, before handing it in. That's two days, not the four I had promised. I delivered exactly what he wanted and I delivered it early.
Before I write a word I always ask the producer or director when they want the next draft by. Even if they say there's no rush, or in your own time, I still push them for a date. That way I can plan ahead and make sure I finish early... every time! I like to impress. I don't skimp on the quality of my writing, I just work harder, faster and for longer each day to deliver an early draft. Director and producers are always pleased when a script lands in their inbox a few days before they were expecting it and then they're more likely to refer me as a writer to others as well.
Make sure you deliver too.