Question

What usually makes it much easier in Europe is that most postdoc positions are without attached teaching, so there is no pressure of that sort when you are considering leaving. Otherwise, everything seem to mainly depend on whether the postdoc position in question is an individual research grant given for your own research proposal or it is a "research associate" position attached to a bigger grant which has already started. In the first case, the starting date is often very flexible, and the leaving date can be easily changed if you have done enough to demonstrate they were paying you money for a good reason; if you want to leave MUCH earlier, say after a year with a 2-year position, then you have to convince them that it is what is crucial for your career growth - but in all cases I know the postdocs did not have problems either. In the second case (research associate), the starting date is usually supposed to be very soon (the grant is given for a fixed period, so they want postdocs to start ASAP to not fail their first year joint report), whereas with leaving early people tend to be rather understanding (but (1) it depends on the grant PI a lot, clearly; (2) my information in that respect is mostly about Ireland and the UK, that might be seriously different in Germany, for example).