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package org.apache.juneau.http;

import org.apache.juneau.http.annotation.*;

/**
 * Represents a parsed <l>Date</l> HTTP request/response header.
 *
 * <p>
 * The date and time that the message was sent (in "HTTP-date" format as defined by RFC 7231).
 *
 * <h5 class='figure'>Example</h5>
 * <p class='bcode w800'>
 *    Date: Tue, 15 Nov 1994 08:12:31 GMT
 * </p>
 *
 * <h5 class='topic'>RFC2616 Specification</h5>
 *
 * The Date general-header field represents the date and time at which the message was originated, having the same
 * semantics as orig-date in RFC 822.
 * The field value is an HTTP-date, as described in section 3.3.1; it MUST be sent in RFC 1123 [8]-date format.
 * <p class='bcode w800'>
 *    Date  = "Date" ":" HTTP-date
 * </p>
 *
 * <p>
 * An example is...
 * <p class='bcode w800'>
 *    Date: Tue, 15 Nov 1994 08:12:31 GMT
 * </p>
 *
 * <p>
 * Origin servers MUST include a Date header field in all responses, except in these cases:
 * <ol>
 *    <li>If the response status code is 100 (Continue) or 101 (Switching Protocols), the response MAY include a Date
 *       header field, at the server's option.
 *    <li>If the response status code conveys a server error, e.g. 500 (Internal Server Error) or 503 (Service
 *       Unavailable), and it is inconvenient or impossible to generate a valid Date.
 *    <li>If the server does not have a clock that can provide a reasonable approximation of the current time, its
 *       responses MUST NOT include a Date header field.
 *       In this case, the rules in section 14.18.1 MUST be followed.
 * </ol>
 *
 * <p>
 * A received message that does not have a Date header field MUST be assigned one by the recipient if the message will
 * be cached by that recipient or gatewayed via a protocol which requires a Date.
 * An HTTP implementation without a clock MUST NOT cache responses without revalidating them on every use.
 * An HTTP cache, especially a shared cache, SHOULD use a mechanism, such as NTP, to synchronize its clock with a
 * reliable external standard.
 *
 * <p>
 * Clients SHOULD only send a Date header field in messages that include an entity-body, as in the case of the PUT and
 * POST requests, and even then it is optional.
 * A client without a clock MUST NOT send a Date header field in a request.
 *
 * <p>
 * The HTTP-date sent in a Date header SHOULD NOT represent a date and time subsequent to the generation of the message.
 * It SHOULD represent the best available approximation of the date and time of message generation, unless the
 * implementation has no means of generating a reasonably accurate date and time.
 * In theory, the date ought to represent the moment just before the entity is generated.
 * In practice, the date can be generated at any time during the message origination without affecting its semantic
 * value.
 *
 * <ul class='seealso'>
 *    <li class='extlink'>{@doc RFC2616}
 * </ul>
 */
@Header(name="Date")
public final class Date extends HeaderDate {

   /**
    * Returns a parsed <c>Date</c> header.
    *
    * @param value The <c>Date</c> header string.
    * @return The parsed <c>Date</c> header, or <jk>null</jk> if the string was null.
    */
   public static Date forString(String value) {
      if (value == null)
         return null;
      return new Date(value);
   }

   private Date(String value) {
      super(value);
   }
}