What is the date format in API

What is the date format in API

Hi,
I have a custom field of type Date/Time (Admin >  Helpdesk Customizer > Incident - Additional Fields  and select Date/Time tab).
Now I want to set value for this field via API.
I found that "31 MAY 2015, 20:52:26" worked.
But, is there any other format available for date fields in REST and Servlet API?

Thank you!
Have a nice day (night)!

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