Expanding on Lizard's post a bit:
The Mac has +<num>h[our]|d[ay]|w[eek]|m[onth]|y[ear] as part of the constructs it understands for date fields, but you don't get to use them in addition to whatever value is already in the field. It's the opposite for the iOS apps, where you get the +1d|w|m buttons to add to the value shown on the date/time picker wheels, but you can't enter expressions. The iOS buttons use the default times if there is nothing in the field when you press them.
If your expression only contains days (or larger) and/or dates, OF will use the default time for the field. So, as I write this at 11:10 AM on 2/15, a start date of "+1d" will get 2/16 12:00 AM and a due date of "+1d" will get 2/16 5:00 PM. However, if you have smaller units (hours) or a time included in the expression, OF will use it. "+1d3h" will get 2/16 2:10 PM for both start and due fields, as will "+27h". Similarly, "+1d 3pm" will give 2/16 3 PM for both start and due fields.
I find myself switching between the two (or using the various scripts) when editing existing dates to get the best of both worlds. Deferring something by a few days or a week is much easier with the iOS interface, but addition of those buttons to the Mac UI would close that gap. Using the scripts on the Mac allow changing multiple items simultaneously, and that overcomes the nuisance factor if there are lots of items to update. Setting up dates in the distant future is much easier on the Mac, of course.