The following dates can be estimated for Tai-Kadai by glottochronology, time depth T being calculated from equations (1) to (3) in the text : Proto-Tai-Kadai would date back to 4000-5000 years ago. Proto-Daic and Li became distinct some 3000-3500 years ago. Proto-Daic split into Be, Lakia, Kam-Sui and Tai about 2000-3000 years ago, with the division of Kam-Tai into Kam-Sui and Tai as the latest development. Proto-Tai began to be differentiated into dialects about 1600 years ago, and the differentiation of Southwestern Tai began about 1000 years ago. Sek seems to be a Northern Tai language, which separated from other Northern Tai languages relatively early on.