}
	}

	// Round ModTime and ignore AccessTime and ChangeTime unless
	// the format is explicitly chosen.
	// This ensures nominal usage of WriteHeader (without specifying the format)
	// does not always result in the PAX format being chosen, which
	// causes a 1KiB increase to every header.
	if tw.hdr.Format == FormatUnknown {
		tw.hdr.ModTime = tw.hdr.ModTime.Round(time.Second)
		tw.hdr.AccessTime = time.Time{}

		}
	}
}

func TestHeaderAllowedFormats(t *testing.T) {
	vectors := []struct {
		header  *Header           // Input header
		paxHdrs map[string]string // Expected PAX headers that may be needed
		formats Format            // Expected formats that can encode the header
	}{{
		header:  &Header{},
		formats: FormatUSTAR | FormatPAX | FormatGNU,
	}, {