Planner reporting

We have a Planner with many buckets and tasks. We have a benchmark requirement that every task must be started within 24 hours of the task creation. Is there a field that we can check to see if the task was started within the 24 hour period? We were thinking a comment, or the checking of a check list item (if there is a datestamp) or the changing from not started to in progress (if there is a datestamp). Any ideas on how to report whether the task has been  worked on? Anyone else have a similar requirement or work around?