Here's what is happening in a nutshell. Date/time values for during DST (last week) are stored to reflect the actual local time. However, date/time values since DST ended (this week) are adding an hour to the local date/time.
Below is REST API value returned for 10/31/2015 at midnight (which falls during DST). The value is offset by 7 hours (07:00:00) because the tenant is in Mountain Standard Time.
And below is the value returned for midnight on 11/3/2015 (which falls after DST has ended). Notice that the offset is now 8 hours (08:00:00), which would actually be 1am in MST. Um, why?