You can view a task's activity and see an audit trail or summary of changes to things like the description, sub tasks created, file uploads, and more. It includes what happened, along with the date.
This gives you a view of what's been going on, in case you'd like to monitor activity or review a list of updates and when they occurred.
View Task Activity
To view a summary of all activity associated with a particular task, scroll to the bottom of a task. Look for the "Comments | Attachments | Activities" filters. Click "Activities."
From there, you will be able to see more details about when (and how) a task was modified.