What to Do About Inaccurate Dates on Letters of Recommendation?
There are a lot of reasons for such inaccuracies. Copy and paste is one. Inattention is another. But none of them that I can think of are your fault, and none seem to be attempts at sabotage. 

As such, I would ignore the issue, especially in written documents in which you have more important and positive things to write. If you are asked about them, your reply can be just that the writer likely made a mistake and if it needs correction, you can probably handle it then. 

Use your motivation letter for precisely that. Don't use it to apologize for the errors of others.