What probably happens in such situation is PM, and the team, has to make some tradeoffs. If the schedule is tight and there's much pressure from the top to deliver anyway the team must sacrifice something in order to get things done on the deadline.
This basically means cutting corners. Now the question is what kind of corners you're going to cut. Commonly it's one of a couple (or both):
- Quality. The team can steal some time from testing in order to build more features.
- Scope. In order to get some functionality done well other part is abandoned at all.
Whatever the team would finally do there are two possible situations and two strategies which may be helpful to the PM.
1. Help to make best possible tradeoffs
It is usually the PM who make the last call on these sacrifices, at least when such decision is made consciously. As a team member the best you can do is to help them to make this decision as good and reasonable as possible. It means feeding them with consequences of each tradeoff, either quality-related or scope-related, so they have possibly best knowledge how things look like from teams perspective.
I'd also throw any ideas which might help to keep the deadline or find arguments to postpone it. It's pretty common than some team members consider a piece of information so obvious they don't even communicate it in any way, while others have no idea about that.
Of course that doesn't mean the decision, which is going to be made, will be satisfactory for the team, as PM has to take into consideration all the technical stuff but also business consequences, e.g. client's reaction to incomplete project, and internal affairs, e.g. stakeholders satisfaction.
2. Get the PM actively looking for solution
The problem can have another root cause as well. PM, instead of making conscious decision about tradeoffs, is choosing hope as a strategy. It meant they hope everything would go fine and the team would deliver before the deadline. If this one is true no wonder why they're stressed.
Anyway in this situation the best thing you can do is to make them aware of the fact that hope is not a strategy and getting them actively tackling the issue, which basically means coming back to the point number 1, once the PM accepts the fact they should actively do something about the problem.
One of techniques which works pretty well here is getting the list of all tasks which have to be done, showing how unlikely it is to get everything done by the deadline and asking about priorities. Then, on the bottom of the list you have your tradeoffs.
By the way, I know PMs who would tell you everything is the top priority, so you may also need to refer to the old rule: if everything has the highest priority, everything has the lowest priority as well and the team is going to choose tradeoffs randomly.