Granite is an igneous rock, meaning it forms from the cooling and solidification of magma or lava. Therefore, there isn't a single specific date when granite formed. Instead, granite formation has been ongoing throughout Earth's history, with different granite bodies forming at different times.

Here's a breakdown:

* Early Earth: Granite started forming very early in Earth's history, possibly as early as 4 billion years ago. These early granites were likely formed from the intense heat and pressure of Earth's formation and early differentiation.

* Continual Formation: Granite continues to form today, albeit at a slower rate than in the past. Granite is still formed through various geological processes like volcanic activity, plate tectonics, and mountain building.

* Age Determination: Geologists can determine the age of specific granite bodies using radioactive dating techniques. This allows them to understand the timeline of granite formation in different regions.

So, while there is no single answer to "when did granite form?", we know that it's been forming for billions of years and continues to do so today.