You'll need to figure out the XPath to the date field in your form and replace the XPath in the below sample with that:
Dim dateNode
Set dateNode = XDocument.DOM.selectSingleNode("/my:myFields/my:dateField")
Will get you the xml node. Then, to get the text:
dateNode.text
So, you could modify what you have to look like:
Dim dateNode
Set dateNode = XDocument.DOM.selectSingleNode("/my:myFields/my:dateField")
Set objOutlook = CreateObject("Outlook.Application")
Set itmAppt = objOutlook.CreateItem(1)
itmAppt.Start = dateNode.text
......
You may want to add some error handling, make sure that there is a value in dateNode.text.If you are using separate fields for the time and date, you'll need to get both values, make sure they aren't blank, and concatenate them. Good luck, and holler if you get stuck.