When a company is acquired, it's often structured as a stock swap where all the founders/investors/employees of the acquisition forfeit their stock in the startup, and in return receive stock in the acquiring company, worth whatever the acquisition price is. It is common for vesting dates on the acquirer's stock to be backdated to the date of employment at the acquisition (which for a founder would be the date of company formation). If they didn't do this, think of how screwed founders & employees would be with every acquisition: you thought you owned $X worth of stock in the company, but actually you own nothing, and you have to work at the acquirer for 4 years to earn back the stock you previously had. (Oftentimes, there might still be a cliff anyway, where you have to work at the acquirer for 6 months or a year anyway to receive the shares you thought you owned.)