Hi Manola,
Thanks for contacting Syncfusion Support.
We would like to let you know that the date object value will be displayed only in the ISO string format in the Mozilla fire fox browser which is the default behavior of the browser. Whenever the date value in local time zone is converted to UTC time zone, the time zone differences between the two-time zones (local machine time zone and UTC time zone) get added to the local date value and the resultant date value in UTC time zone will be displayed. Since you are in UTC+2-time zone, to showcase in ISO format, the date value gets converted to UTC by decreasing two hours of time from the date value (5th September 2018 12:00 AM) which results in pervious date 10 PM (4th September 2018 10:00 PM) in the browser.
However, when this value is again parsed as a date object will result you the value as 5 instead of 4. You can also cross check this with chrome browser as here it will be always displayed as only a date object instead of ISO string as shown in the below screenshot
Also, the reported issue will not occur in normal input DatePicker since the value from the normal HTML DatePicker will return date values in string but not in date object. These conversions will occur only when values are being passed/obtained as a date object.
Please let us know if you need any further assistance.
Regards,
Prem Kumar M