Why can't I write dates in the Expression Editor?

You can't "hard code" a date value directly into the Expression Editor, like this:

When you type a date directly into an Expression Editor like this, what you want to happen is for the date to be treated like a date. But that is not what's happening; the date will more often than not simply be treated as text.

An alternative approach

You can achieve this same goal by creating an Environmental Variable (a variable with a single, "constant" value -- known as a constant variable). You can create Environmental Variables in the App Builder under Settings > Global Configuration:

When you create an Environmental Variable, you can specify it's label, type (e.g. text, date, etc), and value.

Now, you can use your new date Environmental Variable in the Expression Editor.