Get a brief
Before you start a project, take a detailed brief from the client and make sure you understand exactly what their objective is and when the work needs to be delivered. Ensure you understand what you are being asked to deliver and the client's deadline for completion.
Scope out the project
Once you understand the bare bones of the project, work backwards from the deadline date to scope out the project. The project needs to be completed by the client's desired date or a penalty may be incurred, therefore using this date as the finish line will help determine if the deadline is realistic and if the client's outcomes are achievable in the given amount of time.
When confirming a deadline, consider every step of the project, including where you will need extra resource or expertise, as this may need to be arranged well in advance.
Get timescales from staff
When you are happy that you can deliver, make sure the staff involved understand their tasks and the project completion-date. Let them guide you by taking their feedback; ask them for the realistic timescales they need to complete their section of the project.
It is key to be realistic and always keep the client informed. Allow a little extra time for the task to be completed - you never know what may happen when the project is under way.