A day of Bramha consists of 1000 Mahayugas. Each mahayugas has four yugas. They are

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Name of the Yug	Duration in Years
Krita or Satya	1,728,000
Treta	1,296,000
Dvapara	864,000
Kali	432,000

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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.