To use a R date object outside of a script, define a script result variable with data type either "timestamp" or "string". Using "string" has the advantage of formatting the date-time object as you like but when using a timestamp the widget will adjust the representation to the displayed time frame. Use the following code snippets to correctly format the date-objects to meet the correct output format requirements:
Alternatively look at the lubridate package. It offers many functions for handling timestamps and datetime objects in very intuitive way, just remember that lubridate is also expecting a timestamp in seconds and not in milliseconds, so devide the timestamp you get from your data by 1000.