Board Thread:Geometry Dash Gameplay Discussion/@comment-26222768-20150508132712/@comment-25942522-20150508143320

If you are on the custom song screen, click the small gear button on the right hand side. While there, you can see a search field that has the text "start offset" above it. In the search field, you type in the amount of seconds you want to skip of the song. For example: if you want the song to start after 36 seconds, simply types 36 in the search field.

Hope this helped! :)