I suspect the following - The Receiver is shipped with an internal time set, likely to GMT time, that is not impacted when you enter date/time to be your local time. So it is keeping track of the 2 times separately, which helps for example when there is a switch for daylight savings time. The internal/GMT time just keeps ticking without gaining or losing an hour, so that the trend arrows and calibration reminders can stay on track.
When the a sensor reading is sent to the receiver, it records the value, the internal time and the local time. That would be in the export file under the columns SensorInternalTime, SensorDisplayTime and SensorValue.
When you do a calibration using a BG from your meter, it again records 3 columns - InternalTime that meter read was entered, the local time, and the MeterValue.
If you enter events, like exercise, insulin, there is another set of columns for that.
When displayed on the Dexcom software, it would likely show the data using the local time, but that's an interesting thought - I'll have to check what happened during the last time change !