In a way I just fixed my own issue by using today+1 days instead of using workflow reminder field + 1 days.
Even so this might be a bug worth looking into as it will confuse admins who are trying to use the workflow tool creating confusion and perceived unreliability.
Yes it is a custom date field (not datetime).
We resolved the issue by setting the action to add 1 day to todays date and inserting this result into this custom date field
For example, todays date is 22.02.17 so it inserts 23.02.12 into the custom date field
Previously the issue we had was that the custom date field contained 22.02.17 and when we add 1 day to the custom date the result is 23.03.17