Would it be impolite to ask the time of selection decision, although the information is available online?
I would not suggest to write an email that does only ask for the status of the application before the announced time of selection is over—even when the email is totally calm and polite. The program has a schedule, the applicants are supposed to know the schedule.

However, you may have some other good reason to write an email. As Thomas commented to another answer: A good reason to ask for the status would be an offer for another opportunity. You could also try to find another good question to ask (but be sure that it is not already answered in the FAQ or wherever). Also, you could provide some new relevant information, if there is any (e.g. some new grades, completed projects,…).