This date, the only exact one given in the book, specifies the starting point of all the divinely given and commanded words of Moses to the people of Israel. The date appears to be also the day of Moses' death (Deuteronomy 32:48).
This verse also makes clear that God's words were given in a specific historical setting, which adds a certain realness and freshness to the book for also today's reader.
3 Dans la quarantième année, au onzième mois, le premier du mois, Moïse parla aux enfants d'Israël selon tout ce que l'Eternel lui avait ordonné de leur dire.