You can tautologically define terms in that manner but when you are conveying this to others you must ensure that they too accept your definition. If you do not prepend your statements with remarks that detail these redefinitions then you risk miscommunication.

For example, if I sell a drink that I have labeled as "50 year aged Bourbon" but I was referring to the Pawukon calendar (210 days vs. 365) then I would have to make that clear if I did not have the intention of misleading others. General consensus implies that most would expect you to refer to the common definition of "year".