My response would be one of these two, depending on whether it is at all possible for you to make their offered day.
If you just cannot make the offered day, express your great interest in EK, explain briefly to them just how impossible it is for you to make the offered appointment, and indicate all of the other days listed on their website when you could be available.
If you can only make their offered day by moving heaven and earth, express your great interest in EK, explain briefly to them that their proposed appointment time is very difficult for you, but if no other times are available (such as those listed on their website), you will be there.
If EK is unable to provide you with any reasonable flexibility, be forewarned that this suggests they may not be the most sympathetic employer.