There is a difference in user_enrolments timestart and timecreated internally - timestart and timeend indicate when is the enrolment active. The timecreated means when the entry was added to the system. "when student may access course" and "when was student enrolled" into course is not the same.

The current enrolment UI does not show when was student enrolled which seems to be confusing especially for plugins that do not set timestart. Workaround is to display timecreated data when timestart not set in enrol UI.