The strptime() shall behave as specified in the ISO POSIX (2003) with differences as listed below.
The ISO POSIX (2003) specifies fields for which "leading zeros are
permitted but not required"; however, applications shall not expect to
be able to supply more leading zeroes for these fields than would be
implied by the range of the field. Implementations may choose to
either match an input with excess leading zeroes, or treat this as a
non-matching input. For example,
%j has a range of
001 to 366, so 0,
00, 000, 001,
and 045 are acceptable inputs, but inputs such as
0000, 0366 and the like are not.
glibc developers consider it appropriate behavior to forbid excess leading zeroes. When trying to parse a given input against several format strings, forbidding excess leading zeroes could be helpful. For example, if one matches 0011-12-26 against %m-%d-%Y and then against %Y-%m-%d, it seems useful for the first match to fail, as it would be perverse to parse that date as November 12, year 26. The second pattern parses it as December 26, year 11.
The ISO POSIX (2003) is not explicit that an unlimited number of leading zeroes are required, although it may imply this. The LSB explicitly allows implementations to have either behavior. Future versions of this standard may require implementations to forbid excess leading zeroes.
An Interpretation Request is currently pending against ISO POSIX (2003) for this matter.