Usually the way it works is you get signing bonus with your first paycheck, and then you have to return a prorated amount if you leave before the first year. It's possible they negotiated something better, but unlikely.

Amazon avoids this problem altogether by paying the signing bonus 1/52 with each bi-weekly paycheck for the first two years. They also do the proration on five workdays in a week when it's in their favor, but seven days in a week when it's in your favor, so they win either way.