The earliest accounts of Thonmi Sambhota's life were written some four centuries after the events they describe, and even his traditional mission date of 632 is not agreed among them; no source, early or late, gives a birth year, and some scholars question whether a single historical minister stands behind the tradition at all.
What would resolve this A source contemporary with Songtsen Gampo's seventh century reign naming Thonmi Sambhota and a birth year, which no presently known Tibetan or external record supplies.
OpenTibetan history, history of the Tibetan scriptSam van Schaik, Tibet: A History