Thank you Jason,
I thought this could be done with WADA.
Now, I've noticed an interesting thing, as the dates already have validation (that they are a date and that one of them is required) I thought that I would simply add the min and max checks to the existing validations.
It seems that when I open the validations server behaviour and edit the startDate or endDate that the min/max options are greyed out and unavailable.
If I uncheck date, then time automagically selects itself and I can then reselect date and unselect time and then the min/max boxes are available.
This seems like it might be a small bug in the interface?