We plan to make a change to our API in the June 2017 release with regard to the handling of user-defined Date-Time fields.
How the API currently handles UDF Date-Time fields
User-defined Date-Time fields don’t store timezones; however, you can pass a custom Date-Time into the API and include a timezone and the API will accept it. The API ignores the timezone and stores the date, which can cause some unexpected results. For example, if you pass in a time of 3PM PST and a time of 3PM EST, the API will ignore the timezones and treat those values as the same time. The insert or update succeeds, but the data is incorrect.
How the API will handle UDF Date-Time fields starting with the May release
Fields that don't accept timezones won't accept timezones. If you use the API to pass in a custom Date-Time field that includes a timezone, the operation will fail. This prevents the situation where the operation appears to succeed.