Joshua – who had a pretty bad experience as one of Moshe’s 12 spies, only sent two trusted men as spies, and he kept their mission a secret. Rashi employs given facts and basic math as follows: We are told that The Jews mourned 30 days for Moshe and that, after a three-day notice, the Jews crossed the Jordan on the tenth day of Nisan. It is easily extrapolated (a) that Moshe died on the seventh of Adar and (b) that the mission of Joshua’s spies must have occurred during the mourning period for Moshe.