Here's why:
* Proper nouns are specific names of people, places, or things. They are capitalized.
* Common nouns are general names of people, places, or things. They are not capitalized.
The year 1840 is a common noun because it refers to a general concept (a year) and not a specific named entity.
While we might use the year 1840 in specific contexts (like referring to a historical event), it's still not a proper noun.