Are a month old? This is a century old (3,4)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

definition: the Art Deco movement is associated with the 1920s, and was named after a 1925 exhibition [wiki] ART=archaic version of ‘to be’=”Are”, plus DEC (December, “a month”), plus O (old)