1898 Jules Henri Poincaré said that "... we have no direct intuition about the equality of two time intervals."Here is that 1905 Poincare paper. It is really just an abstract of a longer paper. Poincare states that general law of nature in the first paragraph.
1904 Poincaré came very close to special relativity: "... as demanded by the relativity principle the observer cannot know whether he is at rest or in absolute motion."
1905 On June 5, Poincaré finished an article in which he stated that there seems to be a general law of Nature, that it is impossible to demonstrate absolute motion. On June 30, Einstein finished his famous article On the Electrodynamics of Moving Bodies, where he formulated the two postulates of special relativity.
The most important points in that Poincare abstract are that the Lorentz transformations form a group, and that all forces, including electromagnetism and gravity, are affected the same way. Poincare is quite emphatic about both of these points.
Einstein had access to Poincare's abstract before writing his
1905 relativity paper, but he does not quite get these two points, altho he is often credited with them. Here is what they say.
Poincare: "The sum of all these transformations, together with the set of all rotations of space, must form a group; but for this to occur, we need l = 1; so one is forced to suppose l = 1 and this is a consequence that Lorentz has obtained by another way."
Einstein: "we see that such parallel transformations — necessarily — form a group."
Poincare: "Lorentz, in the work quoted, found it necessary to complete his hypothesis by assuming that all forces, whatever their origin, are affected by translation in the same way as electromagnetic forces and, consequently, the effect produced on their components by the Lorentz transformation is still defined by equations (4). It was important to examine this hypothesis more closely and in particular to examine what changes it would require us to make on the law of gravitation."
Einstein: "They suggest rather that, as has already been shown to the first order of small quantities, the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good."
As you can see, Poincare had a deeper understanding of relativity, and he published it first.