All the time values that occur in the API (both as query parameters and as returned payload fields) are ISO8601 timestamps according to UTC timezone e.g. 2018-08-24T08:24:48.652Z.
Average measurements that are calculated for particular period of time (e.g. hourly averages) are returned along with that time period. This period is given as a pair of attributes fromDateTime and tillDateTime and represents a left-closed and right-open time interval [fromDateTime, tillDatetime).
^^^