When does the egg that Professor Elm gives you hatch in "Pokemon HeartGold"?

The Mystery Egg that Professor Elm gives to the player in "Pokémon HeartGold" hatches after the player walks between 2805 and 3059 steps while holding the egg. It hatches into a level 1 Togepi.

In "Pokémon HeartGold," the player initially receives the Mystery Egg from Mr. Pokémon on Route 30, and he delivers it to Professor Elm to study it. Later in the game, after the player earns the Zephyr badge, Professor Elm's aide Zuki in the Violet City Pokémart gives the egg back to the player to hatch it. The Togepi hatched from this egg knows the move Extrasensory, a psychic move that Togepi can only learn otherwise by breeding with a Noctowl or Hoothoot.