Do XML sitemap updates the last modified date of any url automatically ?

I have XML site map in my website but the problem is when i update the website content the url in the site map is not updating the last modified date.

Do i have to generate the site map over and over again when ever i change my website content?

Please help me out ......

from Google Sitemaps http://ift.tt/1Nwgrx6
via IFTTT
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