Two possible Causes:The start/finish times of the files I selected was all good, but when it merged the "start" time of the file was 7 minutes earlier.
Possible Cause 1:
Was this a virtual activity? If so, you need to specify your time zone. Virtual activities do not have GPS coordinates, so Strava doesn't automatically know the time zone in which they were recorded. All activities are recorded in UTC time, which is time at the prime meridian. Also, make sure that you have your LOCATION in Strava set to your home location.
Possible Cause 2:
If you chose the "Randomize ID tag" option when you combined the files (to allow uploading duplicates), the start time could be off by up to 15 minutes. What is this option? Strava uses your activity start time to determine if you are uploading a duplicate activity. If you upload two files with the same start time, then Strava will reject the 2nd upload. My tool gives you the OPTION of randomizing the start time by a few minutes in order to "trick" Strava into thinking that this is a different activity. The reason I give you this option is because people often upload the partial activity and don't want to delete it before uploading the combined file. It basically fools Strava into accepting "duplicate activities". This allows you to have both present at the same time, but you're going to see a little variation in the start time. If you want to avoid this, just remove the original activity and then don't check the randomize ID tag option.