I think "bonus" is a misnomer in this situation. It's part of their compensation. They should have written it to say "performance driven bonus" which would give them latitiude.

It was likely a time-constrained bonus, meaning if they were employed on x date, they would give them the "bonus". It's to keep people there for a predetermined period of time.

A government big enough to give you everything you want, is strong enough to take everything you have.-- Thomas Jefferson