I have a field "When Started Zulu" which has no modifying function and comes out in ISO standard format. I have another field "When Ended Zulu" which takes the star date and adds 1-7 days at random to that field, also specified as ISO format but it is not coming out as ISO. The modifying function is shown below.
field("When Started Zulu") + days(random(1, 7))
example output for "When Started Zulu" - 2019-03-17T17:04:20Z
example output for "When Ended Zulu" - 2019-03-24 17:04:20 UTC