Proposed resolution:

This wording is relative to N4964.

[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]

Option A:

1. Modify [time.format], Table 101 [tab:time.format.spec], as indicated:

   Table 101 — Meaning of conversion specifiers [tab:time.format.spec]

   Specifier	Replacement
   […]
   %C	The year divided by 100 using floored division. If the result is a single decimal digitin range [0, 9], it is prefixed with 0. The modified command %EC produces the locale's alternative representation of the century.
   […]

Option B:

1. Modify [time.format], Table 101 [tab:time.format.spec], as indicated:

   Table 101 — Meaning of conversion specifiers [tab:time.format.spec]

   Specifier	Replacement
   […]
   %C	The year divided by 100 using floored division. If the result is a single decimal digitin range [-9, 9], it is prefixed with 0. The modified command %EC produces the locale's alternative representation of the century.
   […]

Table 101, [tab:time.format.spec] reads:

When the year is in range [-99, -10], it is unclear whether the result (in range [-9, -1]) is considered as a single decimal digit, and there is implementation divergence (llvm/llvm-project#74727).

Daniel:

This issue is somewhat related to LWG 3831.