f.Modified = modified.UTC()

		// If legacy MS-DOS timestamps are set, we can use the delta between
		// the legacy and extended versions to estimate timezone offset.
		//
		// A non-UTC timezone is always used (even if offset is zero).
		// Thus, FileHeader.Modified.Location() == time.UTC is useful for
		// determining whether extended timestamps are present.
		// This is necessary for users that need to do additional time

// The sparse map is stored in the tar header if it's small enough.
// If it's larger than four entries, then one or more extension headers are used
// to store the rest of the sparse map.
//
// The Header.Size does not reflect the size of any extended headers used.
// Thus, this function will read from the raw io.Reader to fetch extra headers.
// This method mutates blk in the process.