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4022. Ambiguity in the formatting of negative years with format specifier %C

Section: 30.12 [time.format] Status: New Submitter: Jiang An Opened: 2023-12-08 Last modified: 2025-10-17

Priority: 3

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Discussion:

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

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 digit, it
is prefixed with 0. The modified command %EC produces the locale's alternative
representation of the century.
[…]

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(i).

[2025-10-17; Reflector poll.]

Set priority to 3 after reflector poll.

Howard intended Option B, i.e. "-09", but Option B could possibly be interpreted to mean "0-9". "Maybe something like the 0 is inserted before the sole digit, or after the sign, if any."

Previous resolution [SUPERSEDED]:

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 30.12 [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 30.12 [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.
    […]

[2025-10-17; Jonathan provides new wording based on Option B]

Proposed resolution:

This wording is relative to N5014.

  1. Modify 30.12 [time.format], Table 133 [tab:time.format.spec], as indicated:

    Table 133 — 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 digit, it
    is prefixed with 0
    in the range [-9, 9] then 0 is inserted before the single digit (after the sign, if present).
    The modified command %EC produces the locale's alternative
    representation of the century.
    […]