Respostas

2013-09-15T22:27:06-03:00
At -  precise time
Ex.: at 10.30 am

In - months, years, centuries and long periods 
EX.: In May, in 1990, in the Ice Age

On - days and dates
EX.: on sunday, on my birthday
2013-09-15T22:54:20-03:00
At, In, On
At = Hours
In = Months
On = Days