Thanks a lot for your fast feedback.
@Axel, the problem is definitely related to the fact that your My Clippings.txt stores the timestamp of the annotation in German.
Duh, yes, parsing back the date/time is even risky in a single language; too many ways to write human readable day/time. Doing so for many languages... even with a mature library it is still asking for trouble. Amazon may decide to "improve" the format at any time.
I guess if I had to program a parser I would opt to keep everything after the last "|" a string. Comparing for equality should still work. Sorting by date won't work, but preserving order of entries may be good enough for most uses.
In the end the question is what is to be / can be done with the notes and highlights. What I am currently looking for is a way to quickly and easily edit a book based on my notes&highlights. Considering that calibre isn't much of an ebook editor, I guess that anyway isn't something your plugin offers, or does it?