limit dates

For a custom order checkout field, I have used the "only allow future dates" option which is great.
What I need is only to allow dates that are at minimum two days out.

For example, todays date is Sept 5, so the customer can only select Sept 7 onwards

There is no option for that.
It would require the development of a custom hikashop plugin in order to override the behavior of the custom field.
If you're interested by such plugin, you can contact jerome at hikashop dot com.