code: func MinNonZeroTime(a, b time.Time) time.Time {
  	if a.IsZero() {
  		return b
  	}
  	if b.IsZero() {
  		return a
  	}
  	return MinTime(a, b)
  }
docs: // MinNonZeroTime returns the earlist time that is not time.Time{}
  // If both a and b are time.Time{}, it returns time.Time{}