Time
"A watch's minute hand moves more quickly than did mine."
There is a large emphasis put on time throughout the tale. The narrator, who is recounting the events, measures time very often in accordance with his actions. It may seem slight, but when taking a close look at the text, symbols and measurements of time can be found everywhere. Much like the narrator's use of the acuteness of his senses and his caution and cleverness, it seems he uses time as a justification of his sanity. Time is important in the tale, because it is important to the narrator as a further indication of his rationality. When he says, "Madmen know nothing", he assumes that because he is able to recount the events so calmly and relative to time, this is an affirmation of his sanity.
Although it is important to the character of the narrator, there is also additional reasoning for Poe's included repetition of time in the story. In his essay on The Tell-Tale Heart, Arthur Robinson, notes the psychological handling of time, which is discussed more under the analysis page here.