"This month": Nissan. You say it is Nissan. But perhaps it was some other month of the year? It is written (Exodus 23:16) "And the festival of the ingathering (Succoth) at the end of the year," (Ibid. 34:22) "And the festival of the ingathering at the turn of the year." In which month is there ingathering and turning (from one solstice to another) and the expiration of a year? Tishrei. After learning that the seventh month is Tishrei, we infer that the first month is Nissan. And even though there is no proof for this, it is intimated in (Esther 3:7) "In the first month, the month of Nissan."