I started a postdoc recently. My advisor has unrealistic expectations of progress. I was hired to implement feature X in their software (among other tasks; this is just the first). I have implemented X in a software written in language A, but have no prior experience with language B that their software is written in or framework C that their software uses. I did not know that they wanted someone to program in language B until the interview, and I explicitly stated that they should look elsewhere if knowing language B is critical as I don't know it. Language B is not known to be an easy language to pick up, though learning it has been on my to-do list for a while.
In multiple meetings recently my advisor has been pushing me to start implementing feature X now as if I've forgotten that's first goal. I know that is the first goal, which I politely stated multiple times, but I've emphasized that I need more time to get familiar with their software, the programming language, and the framework. I don't see myself making the changes I outlined for at least a month, probably longer. And I think my advisor would get angry if I took that long and might even fire me at the end of the probationary period (which is in about a month).
What should I do in this situation? I've tried to emphasize that it will take time for me to get up to speed and this is not avoidable, but this doesn't seem to satisfy my advisor.