This is a hybrid setting between 'daily' email subscriptions and the data set refresh frequency. Though it asks for timezone, the scheduled cache refresh appears to ignore the TZ and emails based on UTC. I receive emails intended for 6am EST at 1am EST (6am UTC). I went a step further and left the TZ as EST and set the report to send out at 11am UTC and recieved the email at 6am EST as intended. I have had to set the dataset to refresh once per day at 11am UTC in order to get the above to email at 6am EST. Otherwise, datasets that refresh in the morning (single updates per day) will only send out that night which is effectively now a full day behind.
My workaround may be complex, but I believe its both an issue if not understanding the Timezone on the settings for datasets page and a lack of robustness in the email subscription settings (ability to send a daily summary at a given time in a TZ).
Wed 11/22/2017 6:02 AM "You’re receiving this email because you subscribed to the ... The image above was generated at 11/22/2017 11:01:54 AM UTC."
Overall, PBI is a fantastic product, very exciting to see the progress being made.