Well, On PL you slather Honey on a tree to attract Pokemon and 6 hours later the tree shakes and a Pokemon appear. There are 21 Honey trees spread throughout Sinnoh.Changing the internal Nintendo DS clock will have no impact on the progression of these 6 hours. You can find heracross by using honey on those trees in platinum. Route 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 218, 221, 222, Eterna Forest, Floaroma Meadow, Fuego Ironworks, Valley Windworks.
Well on this game you have too Headbutt trees to find your Heracross. If a tree does cause a wild Pokémon to appear, encountering a Pokémon for that tree is 100% even if Repel is being used. Likewise, if no Pokémon appears when a tree is Headbutted, no Pokémon will ever appear for that tree. You can find Heracross by headbutting those trees in Heartgold Route 7, 11, 16, 28, 33, 42, 44, 45, 46, 47, Azalea Town, Celadon City, Mt. Silver, Vermillion City.
Ok so my way to find Heracross is in Platinum. You Slather honey on trees before you go to bed. When you wake up at the morning you check those trees and you will find a Heracross sometimes I always find Heracross because of this.
Hope I helped
Headbutting!! Ok just fly to Azealea town and use headbutt on trees here is a video.
How to find Heracross