I am using the REST API for creating a new Task record, including an Activity Date in the format YYYY-MM-DDThh:mm:ssZ. My current timezone is UTC+1.

When submitting a date in the first hour of the day, i.e. for example from 2018-11-19T00:00:00+01:00 to 2018-11-19T00:59:59+01:00, the Activity date is rendered as the previous day on the Task's detail page in my lightning expirience, here 11/18/2018 (Locale: English (United States)). Only when pushing a later date like 2018-11-19T01:00:00+01:00, the Activity Date is correct and shows 11/19/2018.

I then created a custom Date/Time field for the Task and pushed the same dates as above. In that case, every date is rendered correctly, i.e. when pushing 2018-11-19T00:00:00+01:00, it is rendered correctly as 11/19/2018 12:00 AM.

In conclusion, it seems that the Activity Date, which has the field type "Date", is ignoring the timezone, while my custom field, which has the field type "Date/Time", is considering the timezone properly. 
This behavior applies not only for Tasks, but for every object that has fields of type Date or Date/Time.

Is there something that I missed or is it a bug?

I am using the REST API for creating a new Task record, including an Activity Date in the format YYYY-MM-DDThh:mm:ssZ. My current timezone is UTC+1.

When submitting a date in the first hour of the day, i.e. for example from 2018-11-19T00:00:00+01:00 to 2018-11-19T00:59:59+01:00, the Activity date is rendered as the previous day on the Task's detail page in my lightning expirience, here 11/18/2018 (Locale: English (United States)). Only when pushing a later date like 2018-11-19T01:00:00+01:00, the Activity Date is correct and shows 11/19/2018.

I then created a custom Date/Time field for the Task and pushed the same dates as above. In that case, every date is rendered correctly, i.e. when pushing 2018-11-19T00:00:00+01:00, it is rendered correctly as 11/19/2018 12:00 AM.

In conclusion, it seems that the Activity Date, which has the field type "Date", is ignoring the timezone, while my custom field, which has the field type "Date/Time", is considering the timezone properly. 
This behavior applies not only for Tasks, but for every object that has fields of type Date or Date/Time.

Is there something that I missed or is it a bug?