Editor of the JMAP specs here. The JMAP calendar spec is currently just a draft, but now JMAP core and mail are (more or less) finalised and JSCalendar (https://tools.ietf.org/html/draft-ietf-calext-jscalendar-11) is in last call too, we will be looking to take the JMAP Calendar spec through the IETF process in the very near future. Since it is essentially just combing core with the JSCalendar data format, this should be reasonably straight forward.