If you need to get the last time that content was modified in a SharePoint site there is a SPSite and SPWeb property to get that information.

SPSite thisSite = new SPSite();
DateTime.FromFileTime(thisSite.LastContentModifiedDate.ToFileTimeUtc());

This can be helpful in writing a web part or code for displaying site in a web application not modified in a specified time.

Happy Coding!

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