title: ToolTip ignores specified DateTime format specifier
body: <p>I'm struggling with a strange issue regarding <code>ToolTip</code>s in WPF:</p>
  
  <p>First, the code:
  </p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;TextBlock Text="{Binding SelectedItem.Approver.Timestamp, StringFormat='{}{0:g}'}"
             ToolTip="{Binding SelectedItem.Approver.Timestamp, StringFormat='{}{0:f}'}" /&gt;
  </code></pre>
  
  <p>As you can see, the "<a href="https://msdn.microsoft.com/en-us/library/az4se3k1.aspx#Anchor_5" rel="nofollow noreferrer">g format specifier</a>" (e. g. <code>2016-12-24 23:42</code>) is used for the timestamp's display and "<a href="https://msdn.microsoft.com/en-us/library/az4se3k1.aspx#Anchor_3" rel="nofollow noreferrer">f format specifier</a>" (e. g. <code>Saturday, 24. December 2016 23:42</code>) for its <code>ToolTip</code>.</p>
  
  <p>However, what I get is: <code>2016-12-24 23:42</code>. In other words: the <code>ToolTip</code> shows exactly the same format as does the <code>TextBlock</code>.</p>
  
  <p>It gets even weirder when switching the <code>TextBlock</code> to the "f format specifier", too: the <code>ToolTip</code> just stays in the format specified by the "g format specifier".</p>
  
  <p>So, I've got two questions:<br>
  <strong>a) Why is that?</strong><br>
  and<br>
  <strong>b) How to enforce my desired format?</strong></p>