Mankind's technology is still not ready to time travel. You can shift series into the past, but not into the future.

I could tell you a trick how to do it nevertheless, but there is no use of any shift in your code anyway. I guess you just want a crossover. You can find the crossOver function in the manual - there's also an example that just looks as your code should probably look.

http://manual.zorro-trader.com/crossover.htm