Show Last Modified date in your WordPress Posts and Pages.
= Will it requires editing code to show Last Modified date?
Not at all. You can show the revised date by just installing this plugin. Use the Plugin Options to customize the date according to your need.
The "meta revised tag" is used to tell the last modified date & time of a webpage to various web crawlers (Google, Yahoo!, Bing etc.).
The plugin inserts the meta revised tag in the <head> section of your posts and/or pages.
By default, the plugin is already optimized to show the revised meta tag only on posts and pages, and you can opt-in and out anytime to show the tag in them as well. So, you need not to worry about the Homepage and various WordPress Archive Pages.
The plugin make use of your WordPress Date, Time and Time zone (Dashboard > Settings > General). Double check your Date, Time and Time zone before activating this plugin. The idle time for telling the Last Modified time to Search Engines will be UTC.