The control source should not be =Date. That shows that you have a field named "Date" and that needs to change - PERIOD. You cannot expect Access to do things consistently when you use reserved words as object names.
Second, to answer Dennisk's comment - yes =Date() is correct for the default value as it needs to know it is a function you are calling and it will add the brackets anyway if you try not to (unless you have a field named Date which then it will go all wonky).
So, if you created the form using the wizard then you likely also have a control named DATE which can further compound issues. Then, if you have Auto Name Correct enabled, even if you tried to rename it then it will go all wonky on you because you have 2 objects with reserved names which then Access tries to rename all of the references to that text box object but it can't because naming a function that exists can't happen so you just presented it with a paradox.
Moral of the story - DO NOT USE RESERVED WORDS AS OBJECT NAMES.
You may have to turn off Auto Name Correct, rename things, and then import it all into a new, blank database in order to actually fix it.