• A number with no sign, for example, 3 (as in -mtime 3 or -atime 3), means the 24-hour period that ended exactly 3 days ago (in other words, between 96 and 72 hours ago).

• A number with a minus sign (-) refers to the period since that 24-hour period. For example, -3 (as in -mtime -3) is any time between now and 3 days ago (in other words, between 0 and 72 hours ago).

• Naturally, a number with a plus sign (+) refers to the period before that 24-hour period. For example, +3 (as in -mtime +3) is any time more than 3 days ago (in other words, more than 96 hours ago).

Got that? Then you should see that -atime -2 and -atime 1 are both true on files that have been accessed between 48 and 24 hours ago. (-atime -2 is also true on files accessed 24 hours ago or less.)