Move date under the title post

Default setting in blogger.com is the date placement is on the top of the title. You might place the under the post title by following below steps.
Go to Design --> Edit HTML --> Expand Widgets Templates
Press Ctrl+F and a text box will pop up and paste below code in the search box

<b:if cond='data:post.dateHeader'>

You should find code as below.

<b:if cond='data:post.dateHeader'>
<h2 class='date-header'><data:post.dateHeader/></h2>
</b:if>

Remove the full code above and copy paste into notepad
Next step is to locate code below.

<b:if cond='data:post.title'>
<h3 class='post-title entry-title'>
<b:if cond='data:post.link'>
<a expr:href='data:post.link'><data:post.title/></a>
<b:else/>
<b:if cond='data:post.url'>
<a expr:href='data:post.url'><data:post.title/></a>
<b:else/>
<data:post.title/>
</b:if>
</b:if>
</h3>
</b:if>

Paste the code you have in the notepad directly under it.