at shown the perfection means the perfect place, perfect time....for example i will reached there at sharply 11 pm or my car at service station..while in means personal involvement or our relatives involvement...like my parent live in america...i m studying in engineering college,,etc,,,,,

There is a small difference, "at" is used to mean location of something, for example, I shop AT the grocery store, or my car is AT the service-garage. In can also mean location, but it has more of a belonging to it, for example, I am IN a band, my family is IN America.

These are prepositions mean related to grammer.these preposition can be used in different aspects...for example in terms of time, the preposition at is used for the time of day like at 10 oclock, at 11pm etc while in is used for the time of month like in june, in december.....for more information u can check preposition section of english grammer