preposition Working for (a company) or in (a place or situation).
He used to be at Lehman Brothers. Now he's at Merrill Lynch.
preposition Indicating distance or direction relative to the speaker.
Target at five miles. Prepare torpedoes.
preposition Present or taking place during (an event).
Was he at the meeting?
preposition Indicating time of occurrence, especially an instant of time, or a period of time relatively short in context or from the speaker’s perspective.
at six o’clock; at dawn; at closing time; at the age of twelve; at night; at the moment
preposition On (a particular date).
preposition In the direction of; towards; (often implied to be in a hostile or careless manner).
Don’t just talk at someone; really listen to what they have to say.