You could add a validation on your save microflow which checks whether selected leave day is not on the same day.
Example : $SelectedLeaveStartDate=[%CurrentDateTime%]
If above condition is true then Show a validation message ‘Start Date cannot be the same as Today!’
as with numeric values you can use relational expressions to compare dates.
So in your example you could check if there’s already a leave planned (in database) for the selected date with a XPath constraint looking something like:
[$SelectedLeaveDate >= StartDate and $SelectedLeaveDate <= EndDate]
[$SelectedLeaveDate >= StartDate]
[$SelectedLeaveDate <= EndDate]