Add an estimate of the number of hours you think those tasks will take( try to be as honest as possible, don't try to inflate them or keep them low to try to hit a date).
Share that with your manager or whomever is imposing the unrealistic deadlines.
Ask them to help you prioritize these tasks. Show them where you think you will be by the deadline they set, and how much longer you think it will take.
Ok, it's a little more complex than that. In project management people talk about the "triple constraints" of budget, time, and scope.
The relationship between budget and time is not a simple tradeoff. To some extent you can accelerate a software project by increasing the budget, but that extent is limited, maybe you can accelerate the schedule by 30% relative to a "least cost" plan.
Fred Brooks learned these limitations the hard way in the 1960s and he wrote the Mythical Man Month so you don't have to! One problem is that if you add more people to a project, it takes time and attention to onboard them that could otherwise be used to get the project done.
A counter to that is that sometimes buying hardware, software, or services, can greatly accelerate the project. For instance if you are training neural networks on a MacBook, it is probably worth every penny to get a real desktop PC and put a 1080Ti graphics card in it, or to spend some money on cloud computing.
Many managers look at the cost as a function of the deadline, that is, they see the cost of the project as a function of the time you are tied up doing it, so if they can compress the deadline, the cost goes down. (or so they think).
Thus that leads to the "phony deadline" which has no real basis. One problem is that setting out without a realistic plan you are likely to make mistakes which will draw out the project, add to costs, possibly make the project fail.
Most of what I say above is laid out in more detail here:
The flip side of that is the hard deadline, where the job might as well not be done if it is not done on time, for instance, you need to get a grant application in on before a certain day, or you are putting together a demo you are going to show at a trade show on a particular date.
It is important to understand what the actual nature of deadlines that you are up against, what flexibility you have, what impact being late has on the business, etc.
That leaves "scope" as an area with wriggle room. Probably there are some features of the project can be dropped or modified, and management may be able to hit the deadline by dropping features it can afford to drop. This is one big advantage of "agile" methods; if you have something that sorta-kind works at the 25% mark of the project and then you hit the deadline with the most important 80% part of the functionality you are doing better than most people.
Add an estimate of the number of hours you think those tasks will take( try to be as honest as possible, don't try to inflate them or keep them low to try to hit a date).
Share that with your manager or whomever is imposing the unrealistic deadlines.
Ask them to help you prioritize these tasks. Show them where you think you will be by the deadline they set, and how much longer you think it will take.