I ride nearly every day, and I upload my activities to Garmin Connect. Today's ride (it's April 28) is marked as having occurred on April 27. Yesterday's activity is marked on April 27, too. I show two activities on April 24 and April 20 as well. Looking further back, this happened on March 31 and March 8. I show three rides on March 8, two of which are correct. The third was actually done on March 9. But wait - I have a March 9 activity. Oh well, it actually occurred on March 10.
This seems pretty simple - the start and end timestamps are part of the activity data, and the end time and date are part of the file name.
What gives?