https://www.wsj.com/articles/inside-the-marriott-starwood-lo...
According to the article the systems were merged 3 months ago.
"The company resolved one major issue involving elite-night credits earned from credit card spending just last week, more than three months after the integration. That problem left many members in limbo, unsure of how close they were to hitting elite-level thresholds before year’s end."
The intrusion was detected on Starwood's system in September according to the BBC article.
"On September 8, 2018, Marriott received an alert from an internal security tool regarding an attempt to access the Starwood guest reservation database. Marriott quickly engaged leading security experts to help determine what occurred. Marriott learned during the investigation that there had been unauthorized access to the Starwood network since 2014. Marriott recently discovered that an unauthorized party had copied and encrypted information, and took steps towards removing it. On November 19, 2018, Marriott was able to decrypt the information and determined that the contents were from the Starwood guest reservation database."