03_normalize_tick_data.ipynb

Hello all,
I’m new to the community and just started this book. Working through the notebooks I noticed that the author uses 7.5 hours when calculating minutes per trading day. The Itch file times are 09:30 to 16:00, which is 6.5 hours. I searched the errata on the publisher’s website with no luck, since none have been posted, . I searched this forum with no luck. Could someone tell me why the 7.5 was used, or point me in a direction where it is explained? Thanks for your help!