(synopsis as in SUS)
The specification for "strptime" is as specified in the SUSv2 but with the following differences as listed below.
For all fields for which the SUSv2 specifies, "leading zeros are permitted but not required", applications must not expect to be able to supply more leading zeroes 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, or 045 are acceptable inputs, but inputs such as 0000, 0366 and the like are not.
Glibc developers consider forbidding excess leading zeroes to be the correct behavior. 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 SUS 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.