I find the modification date of feed items very important.

RSS does not know the modified date for items.

However, it's possible to use other namespaces providing a modified item property.

Dublin Core and Atom support the modified date for items.

Example:

<dc:modified>2013-09-01T06:07:58+02:00</dc:modified>
<atom:modified>2013-09-01T06:07:58+02:00</atom:modified>

This knowledge is not very common.

Unfortunately, most RSS newsreaders and content management systems (CMS) do not support these modified date tags.

They should be changed.