How to get time String from current_timestamp().toDate() in cumulocity?

Question

In my development over cumulocity using CEL, i assume it will return time String like 2017-01-04T20:55:50.365+08:00 by invoking current_timestamp().toDate() as below, but it doesn't work. How can i get time String value(like 2017-01-04T20:55:50.365+08:00) of current time in cumulocity CEL?

var timestr = current_timestamp().toDate()

Thanks, ZJP