The notebook page dated carefully gives a future reader a place to stand.

A note without a date floats free of everything around it. With one, a future reader can put it in sequence, work out what you knew at the time, and tell a first attempt from a settled conclusion. That reader may be a colleague, a relative, or you in four years with no memory of writing it. Context is what turns a fragment into something usable, and the date is the cheapest piece of context there is.

Put the date at the top before you write anything else. It takes a second, and it is the difference between a record somebody can use and a page nobody can place.