Is the company you are presently at public, and for which there is an options market? If you'll get an extra 6 months' vesting with the cliff, ask the new company to pay you an extra signing bonus NOW, with which you can buy N shares of call options at a strike price of S; N being the number of shares you'll vest in 4 months (6 months minus July and Aug), S being the same as your options' strike price.

If there's no options market, you'll have to guesstimate.

There should be other opportunities beside this offer?