Commentary for Numbers 13:25

וישבו מתור הארץ מקץ ארבעים יום AND THEY RETURNED FROM THE SEARCHING OF THE LAND AT THE END OF FORTY DAYS — But surely it was four hundred Parsangs (Persian miles) by four hundred Parsangs (Megillah 3a). And the journey of an average man is ten Parsangs a day (Pesachim 94a) and consequently it was a distance of forty days from east to west alone, whilst they traveled during that period along its length and breadth! But the fact is that it was manifest before the Holy One, blessed be He, that He would make a decree against them, "a day for a year" (cf. Numbers 14:34), therefore He shortened the road for them (i. e. He made them cover ground rapidly) (Midrash Tanchuma, Sh'lach 8).

מקץ ארבעים יום, “at the end of forty days;” they had commenced their mission on the 29th day of Sivan, and concluded it on the 8th day of the month of Av close to evening. The month of Tammuz in that year had thirty days. (Talmud, tractate Taanit folio 29)