Ignore them and fix it at your own sweet time not working longer than 9-5.
Why you ask? Well look at the big picture.
A manager's ambitious is moving up the corporate ladder. To do this he takes up projects to prove herself. To complete the projects she uses the resources(people) assigned to her. Part of the resources are her core team, that she trusts and will carry with her when she gets promoted. The rest, which is the majority, will stay where they are for the next manager that will come along.
A manager would never give hard time-limit on fixing a bug to his core-team people. The people she trusts. The relationship there is completely different. So we know that you are part of the core team. Hence you have nothing to win career-wise for stressing and working extra hard.
Pushing back, asking questions, and in general being a pain in the ass is also not good career wise, as you may get negative visibility further above.
So the best thing you can do stay invisible, ignore and don't stress about it, and work as fast as you can, while not working more than what you get paid for.
If the bug is really critical and urgent then more resources will be allocated to help you with it.
In most cases however, that deadline is set to multiple people to help with manager's ambition.
So my advise is, relax, don't overwork, but don't rock the boat either.