Today, it is pronounced chén in Mandarin.
However, in Chữ Nôm, this character is used for the sound thì. This is surprising, since most Chữ Nôm characters either borrow the meaning of existing Chinese characters, or the pronunciation of a particular Chinese word.
In chunom.org, it is indicated that 辰 was pronounced like “zhin” during the Tang Dynasty. This doesn’t really correspond with the Vietnamese pronunciation. The meaning of the character doesn’t correspond too. 辰 generally has to do with timekeeping in Chinese, but in Chữ Nôm 辰 is used to write the connector “so, then”.
Were there any developments in the phonology of this particular word during the transition from Old Chinese to Middle Chinese, which resulted in such a divergence? Or is this scenario a random error which cannot be explained using Chinese phonetics?