While not required in an XML sitemap the lastmod element can inform search engines on the last date that the particular page was modified. Here is a typical example.
<lastmod>2018-12-26</lastmod>
The lastmod element in the XML sitemap will support any W3C date or data and time format. Typically either YYYY-MM-DD format or YYYY-MM-DDThh:mm: ssTZD is preferred.
Here is an example of a full entry for reference.
<url>
<loc>https://www.bryan-myers.com/articles/website-notes/update/website-updates-planned-december-2018/</loc>
<changefreq>monthly</changefreq>
<priority>0.8</priority>
<lastmod>2018-12-26</lastmod>
</url>