FrameTiming.frameStartTimestamp

Description

This is the CPU clock time of the time when the frame was started.

Use FrameTimingManager.GetCpuTimerFrequency to convert timestamps to seconds.

using UnityEngine;

class Example
{
    static readonly double k_CpuTimerFrequency = FrameTimingManager.GetCpuTimerFrequency();

    public static double TimestampToMilliseconds(ulong timestamp)
    {
         return (timestamp * 1000.0) / k_CpuTimerFrequency;
    }
}