type Time struct {
    // wall and ext encode the wall time seconds, wall time nanoseconds,
    // and optional monotonic clock reading in nanoseconds.
    //
    // From high to low bit position, wall encodes a 1-bit flag (hasMonotonic),
    // a 33-bit seconds field, and a 30-bit wall time nanoseconds field.
    // The nanoseconds field is in the range [0, 999999999].
    // If the hasMonotonic bit is 0, then the 33-bit field must be zero
    // and the full signed 64-bit wall seconds since Jan 1 year 1 is stored in ext.
    // If the hasMonotonic bit is 1, then the 33-bit field holds a 33-bit
    // unsigned wall seconds since Jan 1 year 1885, and ext holds a
    // signed 64-bit monotonic clock reading, nanoseconds since process start.
    wall uint64
    ext  int64
    ...
 }
復制代碼

 var startNano = 0
 ?
 func init(){
     startNano = runtimeNano()
 }

 func (t Time) Sub(u Time) Duration {
    if t.wall&u.wall&hasMonotonic != 0 {
       te := t.ext
       ue := u.ext
       d := Duration(te - ue)
       if d < 0 && te > ue {
          return maxDuration // t - u is positive out of range
       }
       if d > 0 && te < ue {
          return minDuration // t - u is negative out of range
       }
       return d
    }
    d := Duration(t.sec()-u.sec())*Second + Duration(t.nsec()-u.nsec())
    // Check for overflow or underflow.
    switch {
    case u.Add(d).Equal(t):
       return d // d is correct
    case t.Before(u):
       return minDuration // t - u is negative out of range
    default:
       return maxDuration // t - u is positive out of range
    }
 }