You only provided limited information and therefore it is difficult to make a determination as whether or not this punishment is excessive. What I can share with you is that rarely does being late result in an Article 15, let alone a field grade Article 15.
Regardless you might want to consider appealing the Article 15. You can do so even after the Article 15 process is completed. The punishment cannot be increased, it can be 1). stay the same 2). be reduced 3). or set aside.
You can make a request after the hearing is complete, even if you elected not to appeal during your hearing. However the UCMJ authority does not necessarily have to honor your request but typically they will honor the request.