Wappler 5.8.2, Mac M1
When using the client side formatter for addMinutes, it only calculates negative numbers, and not positive, to force it to calculate the positive numbers, it requires an additional formatter of toNumber(), which makes sense as to get the negative, it would have had to multiply by -1 already, which would have already turned it into a number and not a string.
This Works
dmx.parse("content.sc_staff_shift_attached_events.data.query_attached_to_staff_shift[1].av_start_time_local.replace('Z','').addMinutes(-content.sc_staff_shift_attached_events.data.query_attached_to_staff_shift[1].staff_position[0].pissp_pre_time).formatDate('HH:mm')")
'11:31'
This does not
dmx.parse("content.sc_staff_shift_attached_events.data.query_attached_to_staff_shift[1].av_start_time_local.replace('Z','').addMinutes(content.sc_staff_shift_attached_events.data.query_attached_to_staff_shift[1].staff_position[0].pissp_pre_time).formatDate('HH:mm')")
'12:00'
To force it to work, adjust to
dmx.parse("content.sc_staff_shift_attached_events.data.query_attached_to_staff_shift[1].av_start_time_local.replace('Z','').addMinutes(content.sc_staff_shift_attached_events.data.query_attached_to_staff_shift[1].staff_position[0].pissp_pre_time.toNumber()).formatDate('HH:mm')")
'12:29'
Hope that makes sense, and helps someone else if they are struggling with it.