There can be at least two kind of DateTimes inside metadata: EXIF DateTime tags and XMP DateTime tags. DateTime tags inside XMP have "TimeZone" format. If you see:
"yyyy:mm:dd hh:mm:ssZ" -this means there's no TimeZone offset.

In many cases you can see (for example):
"yyyy:mm:dd hh:mm:ss+01:00"
-meaning: DateTime as set on your camera, plus one hour for TimeZone (as set on your PC, when DNG was created).

This can be very confusing...as Adobe proved in Lightroom -DateTimes inside their Library (database) are totaly unusable.
Hope this was of some help.

Bogdan