Bearing and Hourglass by Piers Anthony

From the back of the book:
When life seemed pointless to Norton, he accepted the position as the Incarnation of Time, even though it meant living backward from present to past. The other seemingly all-powerful incantations of Immortality--Death, Fate, War, and Nature--made him welcome. Even Satan greeted him with gifts. But he soon discovered that the gifts were cunning traps and he had become enmeshed in a complex scheme of the Evil One to destroy all that was good....

This is a book later in a series, but it stands well on its own and I haven't read the rest of the series.