In practice, we've found that only the first two formats seem to support both date and time entry across all three versions of Collector: iOS, Windows, and Android. (If it's mentioned in the Collector documentation somewhere, then we've missed seeing it...)
Once you've picked one of those two formats (m/d/y or d/m/y), then you are able to tick the "Show time" checkbox. (That checkbox is hiding under the drop-down list of formats in Pete Crosier's screen shot above.) You should see something like this in your Configure Attributes window after you select the row with your field, and select one of those formats:
And, checking the box, it becomes...
Now you should be able to pick a date and time in Collector on your iPhone.