Although I am not a native English speaker, I have to disagree with the other answers.
It is obvious: the application restart occurs today. But when does this happen? Did this already happen? (Probably not, past tense should have been used). Is it started and not yet finished? Will it be restarted later? At what time?
The level of precision in specifying time in this case is important. If I have to do something very important using that application, and the application is not working, then I have to adjust my schedule in order to meet the deadline.
So saying that the colleague is "on the autistic spectrum" (written by @EdGrimm) is highly offensive for no reason, considering the situation. In these cases, detailed information is very important.
Moreover, I have trouble with the OP's message itself:
I asked a colleague:
"are we restarting (some application) today"
He writes "asked" and then he provides a simple statement (worded as a question). My point of view is that OP should just try to be more specific when communicating something, whether if transmitting information, or asking something.