title: How can I write an ISO 8601 date with a timezone but no time component
body: <p>An <a href="https://en.wikipedia.org/wiki/ISO_8601#Combined_date_and_time_representations" rel="noreferrer">ISO 8601 datetime</a> with a timezone is formatted like this:</p>
  
  <pre><code>2018-09-07T05:28:42Z
  </code></pre>
  
  <p>However, I need to represent some dates in my system where the precision is days, not seconds, which means that it would be an <a href="https://en.wikipedia.org/wiki/ISO_8601#Calendar_dates" rel="noreferrer">ISO 8601 calendar date</a>. Calendar dates are formatted like this:</p>
  
  <pre><code>2018-09-07
  </code></pre>
  
  <p>In the Wikipedia article about the standard (I don't have access to the standard itself, as you have to pay for that privilege), there is no mention of timezone when discussing dates. It does talk about omitting parts of the time:</p>
  
  <blockquote>
    <p>Either the seconds, or the minutes and seconds, may be omitted from the basic or extended time formats for greater brevity but decreased accuracy: [hh]:[mm], [hh][mm] and [hh] are the resulting reduced accuracy time formats.</p>
  </blockquote>
  
  <p>From this, it seems that you cannot omit hours, so I could not write a calendar date like this:</p>
  
  <pre><code>2018-09-07TZ
  </code></pre>
  
  <p>It appears that the best I can do is truncating down to the hour:</p>
  
  <pre><code>2018-09-07T00Z
  </code></pre>
  
  <p>I don't want to do this if I can avoid it, however, as I am adding more precision to a date than is really there. The date means "sometime during the day of 07 September, 2018 in the UTC timezone", not "sometime during the midnight hour of 07 September, 2018 in the UTC timezone".</p>
  
  <p>Any suggestions?</p>