When negotiating a purchase agreement a buyer has to have a planned/assumed rate of acquisition given their needs and constraints and the manufacturer tries to match up its available production slots to the buyer’s requirements. The final contract has the agreed delivery months in the contract along with any options (usually options are in a side letter to the contract). The contract will probably state something like –
Aircraft 1 – March 2013
Aircraft 2 - May 2013
and so on.
The manufacturers do indeed build to a constant schedule and it requires months of planning with all their suppliers to either increase or decrease the rate of production. When an aircraft is in high demand the buyer has to get in quick and put money down to lock in its required delivery slots. I recall working for a canny operator that had sizeable orders/options for 744's spread over several years but only had deposits down on aircraft due in the next 18 months or so. Demand for the 744 became so great in the early 90’s Boeing demanded non-refundable deposits for aircraft 3 or 4 years away from delivery to lock in the buyers requirements. This caused some hand wringing as the operator was unused to committing that far out in front.
Delivery dates for aircraft a couple of years away can often be deferred if need be