Categorising names of years

[jutfrank]

I started in 1998.

I understand that in 1998 is a prepositional phrase, so I suppose that 1998 is a noun phrase. Could someone confirm that, please?

Secondly, can 1998 be categorised as a noun even though it is not a word in the normal sense? If so, is it a proper noun? If not, what can it be called?

 

[jutfrank]

So do you also see 1998 as the name (proper noun) of the year, Piscean? If written out in full, does that mean we would capitalise it to Nineteen Ninety-Eight?

Also, would you capitalise the name of the century as the Twentieth Century? If not, does that suggest that it isn't a proper noun?