A day of Bramha consists of 1000 Mahayugas. Each mahayugas has four yugas. They are
|Name of the Yug||Duration in Years|
|Krita or Satya||1,728,000|
Now if I have done my sums correctly, each mahayuga is exactly 4,320,000 years (or 4.32 million years) and therefore a day of Brahma is 4.32 billion years.