difficult sentences

[adel87]

dear in this sentence
You must register, schedule and take your exams by June 30, 2010.

why they use by june 30,2010 why they don't use at
what is the différence between by and at
thank you

 

[billmcd]

"By June 30" would indicate that registration must occur from the time the notice is issued (or some date specified prior to June 30) until June 30. "At" should not be used to indicate that registration must occur on a specific date. In this case use "on June 30".