Proposed resolution:

This wording is relative to N4830.

1. Modify Table 99 "Meaning of parse flags [tab:time.parse.spec]" in [time.parse] as indicated:

   Table 99: Meaning of parse flags [tab:time.parse.spec]

   Flag	Parsed value
   […]
   %C	The century as a decimal number. The modified command %NC specifies the maximum number of characters to read. If N is not specified, the default is 2. Leading zeroes are permitted but not required. The modified commands %EC and %OC interprets the locale's alternative representation of the century.
   […]
   %I	The hour (12-hour clock) as a decimal number. The modified command %NI specifies the maximum number of characters to read. If N is not specified, the default is 2. Leading zeroes are permitted but not required. The modified command %OI interprets the locale's alternative representation.
   […]
   %u	The ISO weekday as a decimal number (1-7), where Monday is 1. The modified command %Nu specifies the maximum number of characters to read. If N is not specified, the default is 1. Leading zeroes are permitted but not required. The modified command %Ou interprets the locale's alternative representation.
   %U	The week number of the year as a decimal number. The first Sunday of the year is the first day of week 01. Days of the same year prior to that are in week 00. The modified command %NU specifies the maximum number of characters to read. If N is not specified, the default is 2. Leading zeroes are permitted but not required. The modified command %OU interprets the locale's alternative representation.
   […]
   %W	The week number of the year as a decimal number. The first Monday of the year is the first day of week 01. Days of the same year prior to that are in week 00. The modified command %NW specifies the maximum number of characters to read. If N is not specified, the default is 2. Leading zeroes are permitted but not required. The modified command %OW interprets the locale's alternative representation.
   […]

[ 2020-02 Status to Immediate on Thursday morning in Prague. ]

The current specification of the locale modifiers for the parse flags in "[tab:time.parse.spec] Meaning of parse flags" is inconsistent with the POSIX strptime specification:

• New flags %OC, %Ou are added

• Flags %OI, %OU, %OW are missing

Per Howard's comment:

I only invented in a couple places for these flags, and none of them involved locale-dependent stuff. If we are inconsistent with POSIX/C on this, it's a bug. Rationale, std::get_time is already specified in terms of strptime, and we can't afford to be inventive with locale-dependent things.

Note that, due above, the inconsistency between POSIX strftime specification that supports %Ou and %OV that are not handled by strptime should be (by design) reflected in "[tab:time.format.spec] Meaning of conversion specifiers" and "[tab:time.parse.spec] Meaning of parse flags" tables.

The %d modifier was addressed by LWG 3218.