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<title>Issue 4285: time_get::do_get_date is problematic even after LWG 461</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p>
<h3 id="4285"><a href="lwg-active.html#4285">4285</a>. <code class='backtick'>time_get::do_get_date</code> is problematic even after LWG 461</h3>
<p><b>Section:</b> 28.3.4.6.2.2 <a href="https://wg21.link/locale.time.get.members">[locale.time.get.members]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> S. B. Tam <b>Opened:</b> 2025-06-27 <b>Last modified:</b> 2025-07-01</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Background: <a href="https://github.com/cplusplus/draft/pull/8009">https://github.com/cplusplus/draft/pull/8009</a>
<p/>
LWG <a href="lwg-defects.html#461" title="time_get hard or impossible to implement (Status: CD1)">461</a><sup><a href="https://cplusplus.github.io/LWG/issue461" title="Latest snapshot">(i)</a></sup> changed <code class='backtick'>time_get::do_get_date</code> to parse a set of fixed formats. For example, if 
<code class='backtick'>time_get::date_order()</code> is <code class='backtick'>no_order</code> or <code class='backtick'>mdy</code>, the format parsed by <code class='backtick'>do_get_date</code> is <code class='backtick'>%m/%d/%y</code>.
<p/>
This has several problems:
</p>
<ol>
<li><p>When the resolution of LWG <a href="lwg-defects.html#461" title="time_get hard or impossible to implement (Status: CD1)">461</a><sup><a href="https://cplusplus.github.io/LWG/issue461" title="Latest snapshot">(i)</a></sup> was applied to the draft standard, the slashes were lost. 
This could mislead people into implementing <code class='backtick'>do_get_date</code> using the incorrect formats. Fortunately, none 
of the standard library implementations are affected by this mistake.</p></li>
<li><p>Only 2-digit years are accepted due to the use of <code class='backtick'>%y</code>. This could lead to incorrect parse if 
<code class='backtick'>strftime</code> uses <code class='backtick'>%Y</code> for the locale's date format.</p></li>
<li><p><code class='backtick'>date_order()</code> might need additional locale facets to find out the date order, but it doesn't have an 
<code>ios_base&amp;</code> argument from which to get the locale.</p></li>
<li><p>Many locales do not use any of the specified formats. For example, <code class='backtick'>de_DE</code> uses <code class='backtick'>%d.%m.%Y</code>, <code class='backtick'>zh_CN</code> 
uses <code class='backtick'>%Y年%m月%d日</code>. Although 28.3.4.6.2.3 <a href="https://wg21.link/locale.time.get.virtuals">[locale.time.get.virtuals]</a>/5 gives an implementation the latitude 
to accept additional formats, ambiguity could arise if the locale's format disagrees with <code class='backtick'>date_order()</code>.</p></li>
</ol>
<p>
On POSIX systems, it is possible to query the locale's date format using <code class='backtick'>nl_langinfo(D_FMT)</code>. 
Maybe an implementation should be allowed to use that format <b>instead</b> of the one indicated by <code class='backtick'>date_order()</code>.
</p>


<p id="res-4285"><b>Proposed resolution:</b></p>





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