There are a whole range of answers to this but among other simplifications is that Boeing seem to take less time between first flight and delivery than Airbus do (no, please DON'T start an A vs. B war just now!)
As with any complex manufactured product (and they don't get much more complex than this), despite the best test equipment there will be a range of assembly issues that are only discovered upon first flight. Extent of any rework required is a guess. So that's the first variable; it's why the first aircraft of a type (not just the prototypes) take longer to get right, there's more to find/learn from.
Production (and first flight) is best done in a continuing even stream. But delivery dates will be agreed in the commercial agreement, tied in to all sorts of factors. So aircraft may be ready some days before delivery is due. Some airlines for example notably take delivery of new aircraft (and make the final payment) on the last day of their financial month. Early Airbus (A300) took this to extremes and built aircraft for which they still had no customer at time of completion, and they hung around in some cases for years before delivery. Later regarded as financially mad. Boeing seems never to have done this.
Some commercial agreements may stipulate penalties if the aircraft is not ready on the contracted date. A manufacturer will probably allow a little spare time (bearing in mind paragraph 2 above) just in case. It's a complex calculation offsetting the penalty cost against the cost of borrowing the money for the production of the aircraft, even for a few days, before delivery is due.