Thank you, this works with a little correction.
Do you think is correct?

NOT WORKING
{foreach from=$entries item="dategroup" name="outerloop"}
{foreach from=$dategroup.entries item="entry" name="innerloop"}

{if $startpage && $smarty.innerloop.first && $smarty.outerloop.first}
<div class ...

I think this is pretty simple (but not for me ;) )

I need to add a custom code just after the first entry in the blog homepage (not the first entry of every page).

Like this

POST1
[custom code]
----------------------
POST2
----------------------
POST3
----------------------
---page2----
POST1 ...