John Gill Commentary 2 Samuel 24:8

"So when they had gone to and from through all the land, they came to Jerusalem at the end of nine months and twenty days." — 2 Samuel 24:8 (ASV)

So when they had gone through all the land
Beginning at the east, and from there to the north, and then going about to the west, came to the south, which finished their circuit:

they came to Jerusalem, at the end of nine months and twenty days :
they were ten months wanting ten days in numbering the people; in which they seem to have been very expeditious.