Question

why sun appear two minute before or two minute late from its actual time

Answer 1

This is because of the ATMOSPHERIC REFRACTION

Thats all

Answer 2

The Sun is visible to people on Earth about 2 minutes before the Sun rises above the horizon. Also we are able to see the Sun for about 2 minutes after the Sun goes below the horizon.  This is due to the Refraction of light rays from Sun.

   The layers of air in the atmosphere have different densities. The difference is due to composition, pressure, and temperature. At Sunrise and Sunset the main effect is due to temperature. Hot air is less dense and has a smaller refractive index.

   Due to difference in the refractive indices between the layers, the light rays bend towards the people on the surface of Earth. So we are able to see Sun even though in reality Sun is below the actual line of sight (horizontal line or the horizon).  

   The light rays bend upwards (away from normal) when the density of upper layer is more than that of the lower layer.  The light rays bend downwards (towards the normal) when the density of upper layer is less than the lower layer.