A baseline schedule is a business term for a proposed timeline to indicate the progress of a specific project or contract. More of an estimation than a concrete plan, a baseline schedule should be revamped as soon as any major changes in the scope or circumstances of the project or contract call its accuracy into question. When revamping a baseline schedule, the new schedule completely overwrites the old; in this sense, there can be only one baseline schedule at a time, even though its existence is somewhat of a tenuous thing.
The purpose of baseline schedules is to keep all involved entities — both on the contracting and client sides — up to date on a project's theoretical progress. Often written as a series of goals to be met by certain fixed marker dates, the schedule provides a reasonable pace to ensure the contractor completes the project on time. For example, if contracting for the construction of a new house, a proposed baseline schedule might set the first of May as the date to begin construction, the first of June as the date to begin laying the foundation, and so on.
When calculated by individuals with experience in the field, the schedule should accomplish two objectives. First, it should set both the contractor and the client on the same "page," so that everyone has common expectations. Second, it should provide a suitable meter by which the contractor can readily see if the project is still on schedule.
More of an ideal than an actual obligation, changes to a baseline schedule must be made when major new developments call the original estimates into question. This typically occurs due to changes in the scope of the project. When building a new house, changes to the baseline might need to be made if the clients decide to made substantial additions to the original proposed floor plan. Minor unforeseen delays to the project, such as a late shipment, typically do not invalidate the original schedule.
If and when changes to the baseline are made, the new baseline schedule becomes the only schedule. In other words, after it is changed, it is as though the old schedule never existed. This avoids confusion and keeps everyone's eyes fixed on the end point, not on any problems that have occurred along the way.