Head over to your Project list, and change the Date Filters to "Last Signed".

Then, select the time frame you'd like to search by. For specific dates (Such as last quarter), click Specific Dates. All projects last signed in this time frame will appear!

To view the specific date/time they were last signed, you can add the 'Last Signed' Custom Column to your project list.

Next, click the header of that column to sort from earliest signed, to most recently signed.
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Or, click again to see most recently signed to the earliest signed projects.
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Want to see when a project was first signed? Click here.