An employer may change policies for benefits, such as time for eligibility for health insurance, at will.The above said, IF you left your old job for the new one based in material (significant) part on the representation (claim or promise) that you would be eligible for health insurance in only 30 days, and the new employer knew you would do this and knowing it, nonetheless promised you health insurnace in 30 days to get you to leave your old job for it, then you could potentially hold them to that promise, under the theory of "promissory estoppel." However, to do that, you would have to bring a legal action against them, which may well not be worth it.