I solved it.

Maybe there is an easyer way, but what I did was this:

I broke the conection between the time nodes OutTime and the cached particleShape nodes CurrentTime.

I then connected the time nodes OutTime to an PlusMinusAverage node and the PlusMinusAverage nodes output to the cached particleShape nodes CurrentTime.

Now I can adjust the startingframe by addig or subtracting a value to the PlusMinusAvarage node.

Daniel.