✔ 最佳答案
If you send it to them during office hours, they can know you are very keen to get this post. It also shows that you hope they could go over your application letter immediately and give you a first-hand interview for the post. It is a matter of showing your keen interest in this post if look it from this angle. An 'Active' approach.
If you write them the letter during the leisure time, this will shows you are not so keen but do it during leisure time and is of a trial. A 'passive' approach.