Ok, It's finally solved:
First I tried to find the file named datefilepicker.ftl
inside the struts2-core.jar
but I didn't find anything similar.
So after a few hours trying to find which was the correct template of datetimepicker without any luck, I tried to solve my problem using JQuery because when I was searching for the answer I found out a datepicker tutorial using this library.
There's a plugin called Struts2-JQuery that provides you with ajax functionality and therefore with multiple customizations in different widgets.
It's quite easy to install, I just had to download the correct .jar which for me it was Version 3.5.1 ( jQuery 1.8.3, jQuery UI 1.9.2 ). Once I'd downloaded the file, I just had to paste it in the lib folder inside my project. After doing that, I added this line at the beginning of the .jsp file:
<%@ taglib prefix="sj" uri="/struts-jquery-tags"%>
With these steps I had jquery functionality inside my .jsp file, so the only thing left to do is to add the datepicker widget:
First I chose the jquerytheme I wanted for my widget, just like this:
<head>
<sj:head jquerytheme="flick"/>
...
</head>
And then, I added my datepicker widget:
<sj:datepicker name="start_date" displayFormat="dd-MM-yy" value="today"/>
And that's it.
If you want to resize the datepicker calendar see the answer of this post:
How to resize the jQuery DatePicker control