by Maor Chasen on WordPress.org
Keep track of plugins you activate or deactivate through the dashboard by viewing when each and every one of them was enabled or disabled.

The "Last Activated" column in the plugins dashboard screen
Current features:
While by default the date & time settings are being used, you can always change the date format by filtering ‘pad_date_time_format’. For instance:
Here’s some more info on date & time formatting in WordPress
Feel free to post your feature requests, issues and pull requests to Plugin Activation Date on GitHub.