Rashi on Numbers 10:11/רש"י על במדבר י׳:י״א

1בחדש השני [IN THE SECOND YEAR] IN THE SECOND MONTH [THE CLOUD DEPARTED] — Consequently you must say that they spent at Choreb twelve months less ten days, since on the New Moon of Sivan (the third month) they encamped there (Exodus 19:1) and did not journey until the twentieth of Eyar of the following year.