Ok press menu system setup date/time.... Note almost as suggest by Shemandr the actual date and time comes from the GPS and you cant change it. However you can change the offset for UTC. If you make this correct for your local time it will make the E-80 correct. If you are still in trouble I will find some screenshots fro you but hopefully this puts you on the right track.
PS if you are new to E80 you might find inbuilt tide and current charts usefull. For old technology, and just got rid of my E120 it has some great features, worth reading the manual.