Special relativity. On general relativity he worked from 1907 to 1915.
Speaking of which:
https://en.wikipedia.org/wiki/History_of_general_relativity#...
Seems he had most of the concept nailed down by 1907 but was struggling with the math. So, he worked the module step-by-step only adding gravity in 1915 after learning about Riemannian manifolds.
Special relativity. On general relativity he worked from 1907 to 1915.