The POSIX spec says:
If the −t option is used with the −v option, the standard output format shall be: "%s %u/%u %u %s %d %d:%d %d %s\n", <member mode>, <user ID>, <group ID>, <number of bytes in member>, <abbreviated month>, <day-of-month>, <hour>, <minute>, <year>, <file> where: ... <abbreviated month> Equivalent to the format of the %b conversion specification format in date. <day-of-month> Equivalent to the format of the %e conversion specification format in date. <hour> Equivalent to the format of the %H conversion specification format in date. <minute> Equivalent to the format of the %M conversion specification format in date. <year> Equivalent to the format of the %Y conversion specification format in date.
This actually used to be the format printed by llvm-ar. It was apparently accidentally changed (see r207385 followed by comments in r207387). This makes it conform to GNU ar for easier replacement.