Delta Air Lines is continuing its focus on flight reliability, saying it has operated 200 days this year so far without cancelling any flights in its mainline operation.

That includes Delta-operated flights -- but it does not include Delta Connection flights operated by regional partner carriers, some of which tend to have a higher cancellation rate. Atlanta-based ExpressJet, a Delta Connection carrier, for example, had the highest rate of cancellations among the 12 carriers ranked by the U.S. Bureau of Transportation Statistics in June, July and August.

In the wake of Delta's massive system outage in early August that led to thousands of cancellations, Delta has been working to repair its image and maintain a reputation for reliability.

Back in 2010, Delta had dismal on-time performance, and didn't have a single day without a flight cancellation. It has spent the last several years working to improve. Cancellation-free days are now one of the measures it uses to gauge progress.

According to airline data firm OAG, on-time performance rates for the month of August show Delta ahead of rivals American and United Airlines.