Mods, please delete this if it violates any TOS, but I was listening to the Nancy Grace podcast, "Crime Stories with Nancy Grace," episode from yesterday, July 31, and she was discussing the MT case, and was speaking with MT's cousin, Morgan Collem (sp?).
During the discussion, Nancy asked Morgan if the SC was sent at 10pm, or if the SC was opened by Dalton at 10pm. Morgan states: "To my knowledge, it was sent around 10 pm, just based on what I've heard. It sounds like her boyfriend, Dalton, was getting pretty sleepy, and I believe that he had fallen asleep and then opened the Snapchat the next morning. But the sent time would have been around 10."
Here is a link (snipped for space) to the video upload of the Podcast episode, if you don't have access to the Podcast. The aforementioned interaction begins at 45:23.
I have not heard this information about the SC anywhere else, so it definitely piqued my interest.