There is a way to set this on the attributes (in Window manager highlight the datetime attribute and set the Date Range Operator = After / Before and then Date Range Attribute = the attribute you want this to check against). However it looks from your screen extract that you might be using this on Web Access? If so then I am afraid that this functionality will not work until the next release is out. 7.5 is due for release this month.
You could do a calculation to check whether one date is before the other, and if it isn't the process won't move on. You can do this by adding in a precondition in the process containing a calculation which will check that the date/time field is filled in as should.
This example calculation below will check that the attributes are filled in and it will also check that the start date is before the end date. If this is not true then the action to move the Process forward will not show.
--- import System static def GetAttributeValue(Change): Value = 'False' if Change._StartDateTime is not null and Change._EndDateTime is not null and Change._StartDateTime < Change._EndDateTime: Value = 'True' return Value ---