Cg is completely and totally separate from the rest of Unity (it’s only for graphics cards), so you can’t use C# syntax, Mathf, Time.time or anything. There is a _SinTime but I haven’t looked into how to use it. You probably want to have a look in the manual about writing shaders; there’s a rather large section about it. Although there is much that is still mysterious.
Cg definitely has a sin function (lowercase s). To vary the wave’s phase with time, you should add a property called “phase” (or whatever) to the shader and animate it directly from the Update method of a Unity class. I don’t think there is a good way to vary a property with time directly in the Cg code, but I’d love to be shown otherwise.
There was the WindyGrass.shader + waves.cginc doing sin-wave vertex deformation with an early nature demo package. I’ve used that as a starting point for stuff like this. Attached a zip with the files.
