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#### 8.3.9 Large or Negative Values

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The above sections suggest to use ‘`oldest possible`’ archive format if in doubt. However, sometimes it is not possible. If you attempt to archive a file whose metadata cannot be represented using required format, GNU `tar` will print error message and ignore such a file. You will than have to switch to a format that is able to handle such values. The format summary table (see Formats) will help you to do so.

In particular, when trying to archive files larger than 8GB or with timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16 12:56:31 UTC, you will have to chose between GNU and POSIX archive formats. When considering which format to choose, bear in mind that the GNU format uses two’s-complement base-256 notation to store values that do not fit into standard ustar range. Such archives can generally be read only by a GNU `tar` implementation. Moreover, they sometimes cannot be correctly restored on another hosts even by GNU `tar`. For example, using two’s complement representation for negative time stamps that assumes a signed 32-bit `time_t` generates archives that are not portable to hosts with differing `time_t` representations.

On the other hand, POSIX archives, generally speaking, can be extracted by any tar implementation that understands older ustar format. The only exception are files larger than 8GB.