When it comes to a timeline change in a project, this will causing delay in the project. If you are a project manager, you need to create accurate project timeline, that will help you reach your goals as planned. A Gantt chart timeline need to be followed in the project.
In project management you need to focus on:
The list of tasks to be completed.
The dates on which the tasks need to be complete (like start and end date for example).
The expected duration of each task.
Dependencies between tasks.

A lot of things could be affected in the project if any delay happened, for example, Budget (for example) if you are spending money on pre-clinical study, so you will spend more money if the project delayed.
So, always follow the Gantt Chart.