The FieldLastModifiedDateTime custom field type will display the date/time a specified field was last modified (by looking the the issue history).
The field is defined in a comment of the description of the field:
Note that the field name (case-sensitive) is not the field ID (such as customfield_12345) nor exactly the human-readable field name. The actual field name to use can be found in the changeitem table.
Warning: this custom field might be relatively slow to compute.