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+---
+title: If-Unmodified-Since
+slug: Web/HTTP/Headers/If-Unmodified-Since
+translation_of: Web/HTTP/Headers/If-Unmodified-Since
+---
+<div>{{HTTPSidebar}}</div>
+
+<p>A requisição <strong><code>If-Unmodified-Since</code></strong> do cabeçalho HTTP realiza uma requisição condicional: o servidor enviará de volta o recurso solicitado ou o aceitará no caso de um {{HTTPMethod("POST")}} ou non-{{Glossary("safe")}} método, somente se não tiver sido modificado pela última vez após a data especificada. Se a solicitação tiver sido modificada após a data já especificada, a resposta será um erro {{HTTPStatus("412")}} (Precondition Failed).</p>
+
+<p>A requisição</p>
+
+<p>There are two common use cases:</p>
+
+<ul>
+ <li>In conjunction with non-{{Glossary("safe")}} methods, like {{HTTPMethod("POST")}}, it can be used to implement an <a href="https://en.wikipedia.org/wiki/Optimistic_concurrency_control">optimistic concurrency control</a>, like done by some wikis: editions are rejected if the stored document has been modified since the original has been retrieved.</li>
+ <li>In conjunction with a range request with a {{HTTPHeader("If-Range")}} header, it can be used to ensure that the new fragment requested comes from an unmodified document.</li>
+</ul>
+
+<table class="properties">
+ <tbody>
+  <tr>
+   <th scope="row">Header type</th>
+   <td>{{Glossary("Request header")}}</td>
+  </tr>
+  <tr>
+   <th scope="row">{{Glossary("Forbidden header name")}}</th>
+   <td>no</td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="Sintaxe">Sintaxe</h2>
+
+<pre class="syntaxbox">If-Unmodified-Since: &lt;day-name&gt;, &lt;day&gt; &lt;month&gt; &lt;year&gt; &lt;hour&gt;:&lt;minute&gt;:&lt;second&gt; GMT
+</pre>
+
+<h2 id="Diretivas">Diretivas</h2>
+
+<dl>
+ <dt>&lt;day-name&gt;</dt>
+ <dd>One of "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", or "Sun" (case-sensitive).</dd>
+ <dt>&lt;day&gt;</dt>
+ <dd>2 digit day number, e.g. "04" or "23".</dd>
+ <dt>&lt;month&gt;</dt>
+ <dd>One of "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" (case sensitive).</dd>
+ <dt>&lt;year&gt;</dt>
+ <dd>4 digit year number, e.g. "1990" or "2016".</dd>
+ <dt>&lt;hour&gt;</dt>
+ <dd>2 digit hour number, e.g. "09" or "23".</dd>
+ <dt>&lt;minute&gt;</dt>
+ <dd>2 digit minute number, e.g. "04" or "59".</dd>
+ <dt>&lt;second&gt;</dt>
+ <dd>2 digit second number, e.g. "04" or "59".</dd>
+ <dt><code>GMT</code></dt>
+ <dd>
+ <p>Greenwich Mean Time. HTTP dates are always expressed in GMT, never in local time.</p>
+ </dd>
+</dl>
+
+<h2 id="Exemplos">Exemplos</h2>
+
+<pre>If-Unmodified-Since: Wed, 21 Oct 2015 07:28:00 GMT
+</pre>
+
+<h2 id="Especificações">Especificações</h2>
+
+<table class="standard-table">
+ <tbody>
+  <tr>
+   <th scope="col">Especificação</th>
+   <th scope="col">Título</th>
+  </tr>
+  <tr>
+   <td>{{RFC("7232", "If-Unmodified-Since", "3.4")}}</td>
+   <td>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="Compatibilidade_com_o_Browser">Compatibilidade com o Browser</h2>
+
+<p class="hidden">The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out <a href="https://github.com/mdn/browser-compat-data">https://github.com/mdn/browser-compat-data</a> and send us a pull request.</p>
+
+<p>{{Compat("http.headers.If-Unmodified-Since")}}</p>
+
+<h2 id="Veja_também">Veja também</h2>
+
+<ul>
+ <li>{{HTTPHeader("Last-Modified")}}</li>
+ <li>{{HTTPHeader("If-Modified-Since")}}</li>
+ <li>{{HTTPHeader("If-Match")}}</li>
+ <li>{{HTTPHeader("If-None-Match")}}</li>
+ <li>{{HTTPHeader("If-Range")}}</li>
+ <li>{{HTTPStatus("412")}}<code> Precondition Failed</code></li>
+</ul>