划时代 (huà shí dài) is an adjective meaning 'epoch-making, groundbreaking' — it directly describes something that marks a new era. The idiom 开天辟地 (kāi tiān pì dì) also means 'epoch-making' but is more vivid and literary, evoking the myth of creation; it's used for something utterly unprecedented and earth-shattering. 划时代 is the more straightforward, modern choice for describing landmark events or innovations.