Within Teamwork Desk, you can enable an option to show the last modified information on your help docs.
To enable this, go to the Help Docs area of your site.
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Hover over the Help Docs site you wish to update and select Settings from the dropdown menu.
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Under the Basics subsection, toggle on the option for Show Date Last Modified.
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Once enabled, each article will display the Last Modified details to the top left beneath the help doc name.
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For more information, see: Managing Help Docs Settings