The theory of relativity wasn't proposed in a single moment, but rather developed over time in two distinct parts:

* Special Relativity: First proposed by Albert Einstein in 1905. This theory deals with the relationship between space and time, and how they are affected by motion.

* General Relativity: Published by Einstein in 1915. This theory expands on special relativity, incorporating gravity and its effect on space and time.

So, depending on which theory you are referring to, the answer would be either 1905 or 1915.