Charles Ellicott Commentary Isaiah 16:13

"This is the word that Jehovah spake concerning Moab in time past." — Isaiah 16:13 (ASV)

Since that time. —The phrase is used of an indefinite past, like our “of yore,” or “of old time.” It is variously translated by “hitherto” (2 Samuel 15:34), “from the beginning” (Isaiah 48:3; Isaiah 48:5; Isaiah 48:7). It seems to imply that thus far Isaiah had been in part reproducing the “burden” of an older prophet, or of one given to him to deliver at an earlier date.