+a @command{tar} able to read the good archives they receive.
+
+@node Large or Negative Values
+@subsection Large or Negative Values
+@cindex large values
+@cindex future time stamps
+@cindex negative time stamps
+
+@sc{posix} @command{tar} format uses fixed-sized unsigned octal strings
+to represent numeric values. User and group IDs and device major and
+minor numbers have unsigned 21-bit representations, and file sizes and
+times have unsigned 33-bit representations. @sc{gnu} @command{tar}
+generates @sc{posix} representations when possible, but for values
+outside the @sc{posix} range it generates two's-complement base-256
+strings: uids, gids, and device numbers have signed 57-bit
+representations, and file sizes and times have signed 89-bit
+representations. These representations are an extension to @sc{posix}
+@command{tar} format, so they are not universally portable.
+
+The most common portability problems with out-of-range numeric values
+are large files and future or negative time stamps.
+
+Portable archives should avoid members of 8 GB or larger, as @sc{posix}
+@command{tar} format cannot represent them.
+
+Portable archives should avoid time stamps from the future. @sc{posix}
+@command{tar} format can represent time stamps in the range 1970-01-01
+00:00:00 through 2242-03-16 12:56:31 @sc{utc}. However, many current
+hosts use a signed 32-bit @code{time_t}, or internal time stamp format,
+and cannot represent time stamps after 2038-01-19 03:14:07 @sc{utc}; so
+portable archives must avoid these time stamps for many years to come.
+
+Portable archives should also avoid time stamps before 1970. These time
+stamps are a common @sc{posix} extension but their @code{time_t}
+representations are negative. Many traditional @command{tar}
+implementations generate a two's complement representation for negative
+time stamps that assumes a signed 32-bit @code{time_t}; hence they
+generate archives that are not portable to hosts with differing
+@code{time_t} representations. @sc{gnu} @command{tar} recognizes this
+situation when it is run on host with a signed 32-bit @code{time_t}, but
+it issues a warning, as these time stamps are nonstandard and unportable.
+
+@node Compression