why some git commits are older than their parent?

Question

I was looking at apache-drill commit 3efc2eca and I realized that it is older than its parent commit 8614bae.

I have had a couple of other similar observations. How can I explain them?

Answer 1

Commit history in git is rewritable. There are many ways that that could have happened. The most likely way is that the commit was rebased after it was created.