Hello,
We don't use 'at' with years, so we can't use it here. Usually we use 'at' with times, like 3 o'clock, and we use 'in' with years.
For example:
I'll meet you at 7:30.
I graduated in 2001.
We can also use 'at' with holiday periods like 'at Christmas', 'at New Year', 'at Easter', and also with the expression 'a