Hundreds of 8 letter words include the letter d as the 3th letter. A few of the words include:
add
saddists
dad
ani
dodders, daddies
Dud
dew
din
Letter D
the letter D
The answer to the riddle is the letter "d." It appears at the beginning of the word "time," in the middle of the word "years," and at the end of the word "days." Each part of the riddle cleverly references the position of the letter within those specific words.
Lad