If you are working on ecommerce projects, the best way to manage deadlines is to develop a detailed project workplan at the beginning of the project (at the proposal stage) so that you know just want needs to be done, how many people you need to do it and how long it will take. Don't cut the time down to the bone to please the client as it will only wind up costing you and them time and money.
This requires you to know upfront what the client wants (deliverables, objectives) and how you will get there (your approach and what work needs to be done).
These should all be spelled out in the contract. If there is scope creep (the work changes thus putting pressure on allotted time and resources) you must then go back to the client andre-negotiate time and money.
If this is occuring in your job (non-consulting) then you can adapt these techniques - when you are given an assignment, figure out what you need, how long it will take to get it done and discuss it upfront with your manager. Get buyin at the start and re-negotiate the time if appropriate (and document this). If there is scope creep then you must evaluate the impact and your ability to complete the work in the time at hand with the resources allocated. If theyare insufficient then it must be re-negotiated (and documented).
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unreasonable deadlines