When would I use guarantee instead of guaranty? The dictionary definitions seem pretty much the same. Excepting maybe the noun form of the word.
I have a real world example. A website I'm working on has a 30 day money back guarantee(y?). If you don't like your purchase, within the first 30 days, you can get your money back. Is that a guarantee or a guaranty? If I was going to make a little advertising picture for it, would I say "30 day money back guaranty" or "30 day money back guarantee"?
I am leaning towards guarantee.