Photo's EXIF data Time Zone OffSet Question

I have just taken a trip to Italy took pictures with both my Canon SLR and multiple iPhones.
I copied all images  when I got home and wanted to make sure all TimeStamps are properly sequential.

1. My SLR time was adjusted a few days into the trip to match the time zone I was in. So the first 100 or so pictures were timestamps with Eastern Standard Time (-6 hours from Italy). After I changed the time on the SLR all Date Taken were correct.
2. iPhones time adjusted to the time zone when we landed do all images Date Taken were correct.

So I wanted to make sure that the first 100 or so images taken with my SLR had the correct Date Taken and not be offset by -6 hours.  So what I did was modify the Date Modified +6 hours, but this did not change the Date Taken and I know that this data is EXIF information and cannot be changed the same way I did for date modified.
What I did notice when examine this, Date Taken vs Date Modified on the iPhone EXIF were offset by 6 hours.  So Date Taken was the time in Italy, but the Date Modified is the time in NY.  This data was not manipulated in any way.

Can anyone shed some light on why the iPhone timestamps are offset like that?