Complex sentence: The hacking was an embarrassment to TJ Maxx which discovered the breach in December 2006 and initially believed the intrusion began in May 2006 but further investigation revealed it dated back to July 2005.
Split into simpler sentences: The hacking was an embarrassment to TJ Maxx, which discovered the breach in December 2006. The company initially believed the intrusion began in May 2006, but further investigation revealed breaches dating back to July 2005.