In the used Date-Time format the specified time zone is UTC, defined with a capital letter Z (without any offset):
The date-time values are saved on the server in UTC time, but viewed in Telerik Portal in your local time zone - in your case UTC +2 offset.
If you want to modify the time relative to UTC, remove the Z and add +HH:MM or -HH:MM instead:
This will be saved on the server:
but will be shown in the Portal:
Apr 13, 2017, 10:29AM
Indeed it is not clear that Telerik Portal is viewing the date-time data in your current time zone, therefore I have logged an issue on the matter.
I hope this has helped.
Telerik by Progress