How do I make my block changes after a certain time?

Hello, I'm creating two blocks: one that functions similarly to grass and the other like dirt.
I've been trying to make it function like the vanilla blocks:
- Custom grass becomes dirt after a certain time when detecting that the block above (y  + 1) is solid.
- Custom dirt becomes grass after a certain time when detecting that the block above (y  + 1) is not solid.

I'm trying to do so via NBT timers, but I'm confused about what to do because I just can't seem to get my timer to work.

Any help regarding my question is greatly appreciated. Thank you!