1.在at around 6:00中，around跟about的意思相同。 “Around” is used in front of a number to show that the number is not exact. “Around” means approximately.

2.你说的from under…, until after…是双重介词的用法，这个现象仅限于极少数介词（尤其from为最），例如：

come from behind the door

drop to below 300 dollars

in between the chores

except for the mistake

until after twelve

但at后不可以再跟介词。所以，around不是介词。

3. around是副词，修饰数字6，作状语,表示概数。