I have an item in my mod, it's function is to shoot a projectile and place a block where it lands. I want to add a delay to this after it is shot, and when the delay is over, it set the block back to it's original id. How would I go about adding the delay?

Spawned block should be a TileEntity with tick iterator in update method. 20 tick = sec.

You should also save previous block (before became TileEntity) and after reaching some tick value it would call method that would set block to old (saved) one.

 

There are a lot of good examples for TE (google), but if you really want to learn it's full potential I'd go through code (e.g Furnace).

 

Also - don't forget to save data in NBT, so after unloading world progress is saved.

Could you elaborate a bit more on how to do this, I am still fairly new to forge modding.

Make sa block with a TE. Every tick, add 1 to a counter in the TE. When the counter == 20, set the block to something else.