Always….always get promises, guarantees, agreements…everything work related on paper or mail. If they want you to start working earlier, get it in writing. If they want to make you do more then is in your contract, get it in writing. If they won’t, you should mail them (with delivery and read notification on) that you hereby confirm what was being asked, by whom on what day and what the discussed things were”. BCC your private email address so you have a copy at home.
Always leave a paper trail.